Chapter 6. BIND 9 Configuration Reference
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Chapter 6. BIND 9 Configuration Reference

Table of Contents

Configuration File Elements

Following is a list of elements used throughout the BIND configuration file documentation:

`acl_name`	The name of an `address_match_list` as defined by the acl statement.
`address_match_list`	A list of one or more `ip_addr`, `ip_prefix`, `key_id`, or `acl_name` elements; see the section called “Address Match Lists”.
`masters_list`	A named list of one or more `ip_addr` with optional `key_id` and/or `ip_port`. A `masters_list` may include other `masters_list`s.
`domain_name`	A quoted string which is used as a DNS name; for example, `my.test.domain`.
`namelist`	A list of one or more `domain_name` elements.
`dotted_decimal`	One to four integers valued 0 through 255 separated by dots ("."), such as 123.45.67 or 89.123.45.67.
`ip4_addr`	An IPv4 address with exactly four elements in `dotted_decimal` notation.
`ip6_addr`	An IPv6 address, such as 2001:db8::1234. IPv6-scoped addresses that have ambiguity on their scope zones must be disambiguated by an appropriate zone ID with the percent character ("%") as a delimiter. It is strongly recommended to use string zone names rather than numeric identifiers, to be robust against system configuration changes. However, since there is no standard mapping for such names and identifier values, only interface names as link identifiers are supported, assuming one-to-one mapping between interfaces and links. For example, a link-local address fe80::1 on the link attached to the interface ne0 can be specified as fe80::1%ne0. Note that on most systems link-local addresses always have ambiguity and need to be disambiguated.
`ip_addr`	An `ip4_addr` or `ip6_addr`.
`ip_dscp`	A `number` between 0 and 63, used to select a differentiated services code point (DSCP) value for use with outgoing traffic on operating systems that support DSCP.
`ip_port`	An IP port `number`. The `number` is limited to 0 through 65535, with values below 1024 typically restricted to use by processes running as root. In some cases, an asterisk ("*") character can be used as a placeholder to select a random high-numbered port.
`ip_prefix`	An IP network specified as an `ip_addr`, followed by a slash ("/") and then the number of bits in the netmask. Trailing zeros in an `ip_addr` may be omitted. For example, 127/8 is the network 127.0.0.0 with netmask 255.0.0.0 and 1.2.3.0/28 is network 1.2.3.0 with netmask 255.255.255.240. When specifying a prefix involving a IPv6-scoped address, the scope may be omitted. In that case, the prefix matches packets from any scope.
`key_id`	A `domain_name` representing the name of a shared key, to be used for transaction security.
`key_list`	A list of one or more `key_id`s, separated by semicolons and ending with a semicolon.
`number`	A non-negative 32-bit integer (i.e., a number between 0 and 4294967295, inclusive). Its acceptable value might be further limited by the context in which it is used.
`fixedpoint`	A non-negative real number that can be specified to the nearest one-hundredth. Up to five digits can be specified before a decimal point, and up to two digits after, so the maximum value is 99999.99. Acceptable values might be further limited by the contexts in which they are used.
`path_name`	A quoted string which is used as a pathname, such as `zones/master/my.test.domain`.
`port_list`	A list of an `ip_port` or a port range. A port range is specified in the form of `range` followed by two `ip_port`s, `port_low` and `port_high`, which represents port numbers from `port_low` through `port_high`, inclusive. `port_low` must not be larger than `port_high`. For example, `range 1024 65535` represents ports from 1024 through 65535. In either case an asterisk ("*") character is not allowed as a valid `ip_port`.
`size_spec`	A 64-bit unsigned integer, or the keywords `unlimited` or `default`. Integers may take values 0 <= value <= 18446744073709551615, though certain parameters (such as max-journal-size) may use a more limited range within these extremes. In most cases, setting a value to 0 does not literally mean zero; it means "undefined" or "as big as possible," depending on the context. See the explanations of particular parameters that use `size_spec` for details on how they interpret its use. Numeric values can optionally be followed by a scaling factor: `K` or `k` for kilobytes, `M` or `m` for megabytes, and `G` or `g` for gigabytes, which scale by 1024, 10241024, and 10241024*1024 respectively. `unlimited` generally means "as big as possible," and is usually the best way to safely set a very large number. `default` uses the limit that was in force when the server was started.
`size_or_percent`	A `size_spec` or integer value followed by "%" to represent percent. The behavior is exactly the same as `size_spec`, but `size_or_percent` also allows specifying a positive integer value followed by the "%"" sign to represent percent.
`yes_or_no`	Either `yes` or `no`. The words `true` and `false` are also accepted, as are the numbers `1` and `0`.
`dialup_option`	One of `yes`, `no`, `notify`, `notify-passive`, `refresh`, or `passive`. When used in a zone, `notify-passive`, `refresh`, and `passive` are restricted to secondary and stub zones.

Address Match Lists

Syntax

address_match_list = address_match_list_element ; ...

address_match_list_element = [ ! ] ( ip_address | ip_prefix |
     key key_id | acl_name | { address_match_list } )

Definition and Usage

Address match lists are primarily used to determine access control for various server operations. They are also used in the listen-on and sortlist statements. The elements which constitute an address match list can be any of the following:

an IP address (IPv4 or IPv6)
an IP prefix (in "/" notation)
a key ID, as defined by the key statement
the name of an address match list defined with the acl statement
a nested address match list enclosed in braces

Elements can be negated with a leading exclamation mark ("!"), and the match list names "any", "none", "localhost", and "localnets" are predefined. More information on those names can be found in the description of the acl statement.

The addition of the key clause made the name of this syntactic element something of a misnomer, since security keys can be used to validate access without regard to a host or network address. Nonetheless, the term "address match list" is still used throughout the documentation.

When a given IP address or prefix is compared to an address match list, the comparison takes place in approximately O(1) time. However, key comparisons require that the list of keys be traversed until a matching key is found, and therefore may be somewhat slower.

The interpretation of a match depends on whether the list is being used for access control, defining listen-on ports, or in a sortlist, and whether the element was negated.

When used as an access control list, a non-negated match allows access and a negated match denies access. If there is no match, access is denied. The clauses allow-notify, allow-recursion, allow-recursion-on, allow-query, allow-query-on, allow-query-cache, allow-query-cache-on, allow-transfer, allow-update, allow-update-forwarding, blackhole, and keep-response-order all use address match lists. Similarly, the listen-on option causes the server to refuse queries on any of the machine's addresses which do not match the list.

Order of insertion is significant. If more than one element in an ACL is found to match a given IP address or prefix, preference is given to the one that came first in the ACL definition. Because of this first-match behavior, an element that defines a subset of another element in the list should come before the broader element, regardless of whether either is negated. For example, in 1.2.3/24; ! 1.2.3.13; the 1.2.3.13 element is completely useless because the algorithm matches any lookup for 1.2.3.13 to the 1.2.3/24 element. Using ! 1.2.3.13; 1.2.3/24 fixes that problem by blocking 1.2.3.13 via the negation, but all other 1.2.3.* hosts pass through.

Comment Syntax

The BIND 9 comment syntax allows comments to appear anywhere that whitespace may appear in a BIND configuration file. To appeal to programmers of all kinds, they can be written in the C, C++, or shell/perl style.

Syntax

/* This is a BIND comment as in C */

// This is a BIND comment as in C++

# This is a BIND comment as in common Unix shells
# and perl

Definition and Usage

Comments may appear anywhere that whitespace may appear in a BIND configuration file.

C-style comments start with the two characters /* (slash, star) and end with */ (star, slash). Because they are completely delimited with these characters, they can be used to comment only a portion of a line or to span multiple lines.

C-style comments cannot be nested. For example, the following is not valid because the entire comment ends with the first */:

/* This is the start of a comment.
   This is still part of the comment.
/* This is an incorrect attempt at nesting a comment. */
   This is no longer in any comment. */

C++-style comments start with the two characters // (slash, slash) and continue to the end of the physical line. They cannot be continued across multiple physical lines; to have one logical comment span multiple lines, each line must use the // pair. For example:

// This is the start of a comment.  The next line
// is a new comment, even though it is logically
// part of the previous comment.

Shell-style (or perl-style) comments start with the character # (number sign) and continue to the end of the physical line, as in C++ comments. For example:

# This is the start of a comment.  The next line
# is a new comment, even though it is logically
# part of the previous comment.

Warning

The semicolon (";") character cannot start a comment, unlike in a zone file. The semicolon indicates the end of a configuration statement.

Configuration File Grammar

A BIND 9 configuration consists of statements and comments. Statements end with a semicolon; statements and comments are the only elements that can appear without enclosing braces. Many statements contain a block of sub-statements, which are also terminated with a semicolon.

The following statements are supported:

acl	Defines a named IP address matching list, for access control and other uses.
controls	Declares control channels to be used by the rndc utility.
include	Includes a file.
key	Specifies key information for use in authentication and authorization using TSIG.
logging	Specifies what information the server logs and where the log messages are sent.
lwres	Configures named to also act as a lightweight resolver daemon (lwresd).
masters	Defines a named list of primary servers for inclusion in stub and secondary zones' masters or also-notify lists.
options	Controls global server configuration options and sets defaults for other statements.
server	Sets certain configuration options on a per-server basis.
statistics-channels	Declares communication channels to get access to named statistics.
trusted-keys	Defines trusted DNSSEC keys.
managed-keys	Lists DNSSEC keys to be kept up-to-date using RFC 5011 trust anchor maintenance.
view	Defines a view.
zone	Defines a zone.

The logging and options statements may only occur once per configuration.

acl Statement Grammar

acl string { address_match_element; ... };

acl Statement Definition and Usage

The acl statement assigns a symbolic name to an address match list. It gets its name from one of the primary uses of address match lists: Access Control Lists (ACLs).

The following ACLs are built-in:

any	Matches all hosts.
none	Matches no hosts.
localhost	Matches the IPv4 and IPv6 addresses of all network interfaces on the system. When addresses are added or removed, the localhost ACL element is updated to reflect the changes.
localnets	Matches any host on an IPv4 or IPv6 network for which the system has an interface. When addresses are added or removed, the localnets ACL element is updated to reflect the changes. Some systems do not provide a way to determine the prefix lengths of local IPv6 addresses; in such cases, localnets only matches the local IPv6 addresses, just like localhost.

controls Statement Grammar

controls {
	inet ( ipv4_address | ipv6_address |
	    * ) [ port ( integer | * ) ] allow
	    { address_match_element; ... } [
	    keys { string; ... } ] [ read-only
	    boolean ];
	unix quoted_string perm integer
	    owner integer group integer [
	    keys { string; ... } ] [ read-only
	    boolean ];
};

controls Statement Definition and Usage

The controls statement declares control channels to be used by system administrators to manage the operation of the name server. These control channels are used by the rndc utility to send commands to and retrieve non-DNS results from a name server.

An inet control channel is a TCP socket listening at the specified ip_port on the specified ip_addr, which can be an IPv4 or IPv6 address. An ip_addr of * (asterisk) is interpreted as the IPv4 wildcard address; connections are accepted on any of the system's IPv4 addresses. To listen on the IPv6 wildcard address, use an ip_addr of ::. If rndc is only used on the local host, using the loopback address (127.0.0.1 or ::1) is recommended for maximum security.

If no port is specified, port 953 is used. The asterisk "*" cannot be used for ip_port.

The ability to issue commands over the control channel is restricted by the allow and keys clauses. Connections to the control channel are permitted based on the address_match_list. This is for simple IP address-based filtering only; any key_id elements of the address_match_list are ignored.

A unix control channel is a Unix domain socket listening at the specified path in the file system. Access to the socket is specified by the perm, owner, and group clauses. Note on some platforms (SunOS and Solaris), the permissions (perm) are applied to the parent directory as the permissions on the socket itself are ignored.

The primary authorization mechanism of the command channel is the key_list, which contains a list of key_ids. Each key_id in the key_list is authorized to execute commands over the control channel. See Remote Name Daemon Control application in the section called “Administrative Tools”) for information about configuring keys in rndc.

If the read-only clause is enabled, the control channel is limited to the following set of read-only commands: nta -dump, null, status, showzone, testgen, and zonestatus. By default, read-only is not enabled and the control channel allows read-write access.

If no controls statement is present, named sets up a default control channel listening on the loopback address 127.0.0.1 and its IPv6 counterpart ::1. In this case, and also when the controls statement is present but does not have a keys clause, named attempts to load the command channel key from the file rndc.key in /etc (or whatever sysconfdir was specified when BIND was built). To create an rndc.key file, run rndc-confgen -a.

The key name and the size of the secret cannot be easily changed; if it is desirable to change those things, make a rndc.conf with a custom key. The rndc.key file also has its permissions set such that only the owner of the file (the user that named is running as) can access it. For greater flexibility in allowing other users to access rndc commands, create a rndc.conf file and make it group-readable by a group that contains the users who should have access.

To disable the command channel, use an empty controls statement: controls { };.

include Statement Grammar

include filename;

include Statement Definition and Usage

The include statement inserts the specified file at the point where the include statement is encountered. The include statement facilitates the administration of configuration files by permitting the reading or writing of some things but not others. For example, the statement could include private keys that are readable only by the name server.

key Statement Grammar

key string {
	algorithm string;
	secret string;
};

key Statement Definition and Usage

The key statement defines a shared secret key for use with TSIG (see the section called “TSIG”) or the command channel (see the section called “controls Statement Definition and Usage”).

The key statement can occur at the top level of the configuration file or inside a view statement. Keys defined in top-level key statements can be used in all views. Keys intended for use in a controls statement (see the section called “controls Statement Definition and Usage”) must be defined at the top level.

The key_id, also known as the key name, is a domain name that uniquely identifies the key. It can be used in a server statement to cause requests sent to that server to be signed with this key, or in address match lists to verify that incoming requests have been signed with a key matching this name, algorithm, and secret.

The algorithm_id is a string that specifies a security/authentication algorithm. The named server supports hmac-md5, hmac-sha1, hmac-sha224, hmac-sha256, hmac-sha384, and hmac-sha512 TSIG authentication. Truncated hashes are supported by appending the minimum number of required bits preceded by a dash, e.g., hmac-sha1-80. The secret_string is the secret to be used by the algorithm, and is treated as a Base64-encoded string.

logging Statement Grammar

logging {
	category string { string; ... };
	channel string {
		buffered boolean;
		file quoted_string [ versions ( "unlimited" | integer )
		    ] [ size size ];
		null;
		print-category boolean;
		print-severity boolean;
		print-time boolean;
		severity log_severity;
		stderr;
		syslog [ syslog_facility ];
	};
};

logging Statement Definition and Usage

The logging statement configures a wide variety of logging options for the name server. Its channel phrase associates output methods, format options, and severity levels with a name that can then be used with the category phrase to select how various classes of messages are logged.

Only one logging statement is used to define as many channels and categories as desired. If there is no logging statement, the logging configuration is:

logging {
     category default { default_syslog; default_debug; };
     category unmatched { null; };
};

If named is started with the -L option, it logs to the specified file at startup, instead of using syslog. In this case the logging configuration is:

logging {
     category default { default_logfile; default_debug; };
     category unmatched { null; };
};

The logging configuration is only established when the entire configuration file has been parsed. When the server starts up, all logging messages regarding syntax errors in the configuration file go to the default channels, or to standard error if the -g option was specified.

The channel Phrase

All log output goes to one or more channels; there is no limit to the number of channels that can be created.

Every channel definition must include a destination clause that says whether messages selected for the channel go to a file, go to a particular syslog facility, go to the standard error stream, or are discarded. The definition can optionally also limit the message severity level that is accepted by the channel (the default is info), and whether to include a named-generated time stamp, the category name, and/or the severity level (the default is not to include any).

The null destination clause causes all messages sent to the channel to be discarded; in that case, other options for the channel are meaningless.

The file destination clause directs the channel to a disk file. It can include limitations both on how large the file is allowed to become, and on how many versions of the file are saved each time the file is opened.

If the versions log file option is used, then named retains that many backup versions of the file by renaming them when opening. For example, to keep three old versions of the file lamers.log, just before it is opened lamers.log.1 is renamed to lamers.log.2, lamers.log.0 is renamed to lamers.log.1, and lamers.log is renamed to lamers.log.0. The versions unlimited option can be set to not limit the number of versions. If a size option is associated with the log file, then renaming is only done when the file being opened exceeds the indicated size. No backup versions are kept by default; any existing log file is simply appended.

The size option for files is used to limit log growth. If the file ever exceeds the size, then named stops writing to the file unless it also has a versions option associated with it. If backup versions are kept, the files are rolled as described above and a new one begun. If there is no versions option, no more data is written to the log until some out-of-band mechanism removes or truncates the log to less than the maximum size. The default behavior is not to limit the size of the file.

Here is an example using the size and versions options:

channel an_example_channel {
    file "example.log" versions 3 size 20m;
    print-time yes;
    print-category yes;
};

The syslog destination clause directs the channel to the system log. Its argument is a syslog facility as described in the syslog man page. Known facilities are kern, user, mail, daemon, auth, syslog, lpr, news, uucp, cron, authpriv, ftp, local0, local1, local2, local3, local4, local5, local6, and local7; however, not all facilities are supported on all operating systems. How syslog handles messages sent to this facility is described in the syslog.conf man page. On a system which uses a very old version of syslog, which only uses two arguments to the openlog() function, then this clause is silently ignored.

On Windows machines, syslog messages are directed to the EventViewer.

The severity clause works like syslog's "priorities," except that they can also be used when writing straight to a file rather than using syslog. Messages which are not at least of the severity level given are not selected for the channel; messages of higher severity levels are accepted.

When using syslog, the syslog.conf priorities also determine what eventually passes through. For example, defining a channel facility and severity as daemon and debug, but only logging daemon.warning via syslog.conf, causes messages of severity info and notice to be dropped. If the situation were reversed, with named writing messages of only warning or higher, then syslogd would print all messages it received from the channel.

The stderr destination clause directs the channel to the server's standard error stream. This is intended for use when the server is running as a foreground process, as when debugging a configuration, for example.

The server can supply extensive debugging information when it is in debugging mode. If the server's global debug level is greater than zero, debugging mode is active. The global debug level is set either by starting the named server with the -d flag followed by a positive integer, or by running rndc trace. The global debug level can be set to zero, and debugging mode turned off, by running rndc notrace. All debugging messages in the server have a debug level; higher debug levels give more detailed output. Channels that specify a specific debug severity, for example:

channel specific_debug_level {
    file "foo";
    severity debug 3;
};

get debugging output of level 3 or less any time the server is in debugging mode, regardless of the global debugging level. Channels with dynamic severity use the server's global debug level to determine what messages to print.

If print-time is set to yes, then the date and time are logged. print-time may be specified for a syslog channel, but is usually unnecessary since syslog also logs the date and time. If print-category is set to yes, then the category of the message is logged as well. Finally, if print-severity is set, then the severity level of the message is logged. The print- options may be used in any combination, and are always printed in the following order: time, category, severity. Here is an example where all three print- options are on:

28-Feb-2000 15:05:32.863 general: notice: running

If buffered has been turned on, the output to files is not flushed after each log entry. By default all log messages are flushed.

There are four predefined channels that are used for named's default logging, as follows. If named is started with the -L, then a fifth channel, default_logfile, is added. How they are used is described in the section called “The category Phrase”.

channel default_syslog {
    // send to syslog's daemon facility
    syslog daemon;
    // only send priority info and higher
    severity info;
};

channel default_debug {
    // write to named.run in the working directory
    // Note: stderr is used instead of "named.run" if
    // the server is started with the '-g' option.
    file "named.run";
    // log at the server's current debug level
    severity dynamic;
};

channel default_stderr {
    // writes to stderr
    stderr;
    // only send priority info and higher
    severity info;
};

channel null {
   // toss anything sent to this channel
   null;
};

channel default_logfile {
    // this channel is only present if named is
    // started with the -L option, whose argument
    // provides the file name
    file "...";
    // log at the server's current debug level
    severity dynamic;
};

The default_debug channel has the special property that it only produces output when the server's debug level is non-zero. It normally writes to a file called named.run in the server's working directory.

For security reasons, when the -u command-line option is used, the named.run file is created only after named has changed to the new UID, and any debug output generated while named is starting - and still running as root - is discarded. To capture this output, run the server with the -L option to specify a default logfile, or the -g option to log to standard error which can be redirected to a file.

Once a channel is defined, it cannot be redefined. The built-in channels cannot be altered directly, but the default logging can be modified by pointing categories at defined channels.

The category Phrase

There are many categories, so desired logs can be sent anywhere while unwanted logs are ignored. If a list of channels is not specified for a category, log messages in that category are sent to the default category instead. If no default category is specified, the following "default default" is used:

category default { default_syslog; default_debug; };

If named is started with the -L option, the default category is:

category default { default_logfile; default_debug; };

As an example, let's say a user wants to log security events to a file, but also wants to keep the default logging behavior. They would specify the following:

channel my_security_channel {
    file "my_security_file";
    severity info;
};
category security {
    my_security_channel;
    default_syslog;
    default_debug;
};

To discard all messages in a category, specify the null channel:

category xfer-out { null; };
category notify { null; };

The following are the available categories and brief descriptions of the types of log information they contain. More categories may be added in future BIND releases.

client	Processing of client requests.
cname	Name servers that are skipped for being a CNAME rather than A/AAAA records.
config	Configuration file parsing and processing.
database	Messages relating to the databases used internally by the name server to store zone and cache data.
default	Logging options for those categories where no specific configuration has been defined.
delegation-only	Queries that have been forced to NXDOMAIN as the result of a delegation-only zone or a delegation-only in a forward, hint, or stub zone declaration.
dispatch	Dispatching of incoming packets to the server modules where they are to be processed.
dnssec	DNSSEC and TSIG protocol processing.
dnstap	The "dnstap" DNS traffic capture system.
edns-disabled	Log queries that have been forced to use plain DNS due to timeouts. This is often due to the remote servers not being RFC 1034-compliant (not always returning FORMERR or similar to EDNS queries and other extensions to the DNS when they are not understood). In other words, this is targeted at servers that fail to respond to DNS queries that they don't understand. Note: the log message can also be due to packet loss. Before reporting servers for non-RFC 1034 compliance they should be re-tested to determine the nature of the non-compliance. This testing should prevent or reduce the number of false-positive reports. Note: eventually named will have to stop treating such timeouts as due to RFC 1034 non-compliance and start treating it as plain packet loss. Falsely classifying packet loss as due to RFC 1034 non-compliance impacts DNSSEC validation, which requires EDNS for the DNSSEC records to be returned.
general	Catch-all for many things that still are not classified into categories.
lame-servers	Misconfigurations in remote servers, discovered by BIND 9 when trying to query those servers during resolution.
network	Network operations.
notify	The NOTIFY protocol.
queries	Location where queries should be logged. At startup, specifying the category queries also enables query logging unless querylog option has been specified. The query log entry first reports a client object identifier in @0x<hexadecimal-number> format. Next, it reports the client's IP address and port number, and the query name, class, and type. Next, it reports whether the Recursion Desired flag was set (+ if set, - if not set), whether the query was signed (S), whether EDNS was in use along with the EDNS version number (E(#)), whether TCP was used (T), whether DO (DNSSEC Ok) was set (D), whether CD (Checking Disabled) was set (C), whether a valid DNS Server COOKIE was received (V), and whether a DNS COOKIE option without a valid Server COOKIE was present (K). After this, the destination address the query was sent to is reported. `client 127.0.0.1#62536 (www.example.com): query: www.example.com IN AAAA +SE` `client ::1#62537 (www.example.net): query: www.example.net IN AAAA -SE` The first part of this log message, showing the client address/port number and query name, is repeated in all subsequent log messages related to the same query.
query-errors	Information about queries that resulted in some failure.
rate-limit	The start, periodic, and final notices of the rate limiting of a stream of responses are logged at info severity in this category. These messages include a hash value of the domain name of the response and the name itself, except when there is insufficient memory to record the name for the final notice. The final notice is normally delayed until about one minute after rate limiting stops. A lack of memory can hurry the final notice, which is indicated by an initial asterisk (). Various internal events are logged at debug level 1 and higher. Rate limiting of individual requests is logged in the query-errors* category.
resolver	DNS resolution, such as the recursive lookups performed on behalf of clients by a caching name server.
rpz	Information about errors in response policy zone files, rewritten responses, and, at the highest debug levels, mere rewriting attempts.
security	Approval and denial of requests.
spill	Queries that have been terminated, either by dropping or responding with SERVFAIL, as a result of a fetchlimit quota being exceeded.
trust-anchor-telemetry	Trust-anchor-telemetry requests received by named.
unmatched	Messages that named was unable to determine the class of, or for which there was no matching view. A one-line summary is also logged to the client category. This category is best sent to a file or stderr; by default it is sent to the null channel.
update	Dynamic updates.
update-security	Approval and denial of update requests.
xfer-in	Zone transfers the server is receiving.
xfer-out	Zone transfers the server is sending.

The query-errors Category

The query-errors category is used to indicate why and how specific queries resulted in responses which indicate an error. Normally, these messages will be logged at debug logging levels; note, however, that if query logging is active, some are logged at info. The logging levels are described below:

At debug level 1 or higher - or at info, when query logging is active - each response with response code SERVFAIL is logged as follows:

client 127.0.0.1#61502: query failed (SERVFAIL) for www.example.com/IN/AAAA at query.c:3880

This means an error resulting in SERVFAIL was detected at line 3880 of source file query.c. Log messages of this level are particularly helpful in identifying the cause of SERVFAIL for an authoritative server.

At debug level 2 or higher, detailed context information about recursive resolutions that resulted in SERVFAIL is logged. The log message looks like this:

fetch completed at resolver.c:2970 for www.example.com/A
in 10.000183: timed out/success [domain:example.com,
referral:2,restart:7,qrysent:8,timeout:5,lame:0,quota:0,neterr:0,
badresp:1,adberr:0,findfail:0,valfail:0]

The first part before the colon shows that a recursive resolution for AAAA records of www.example.com completed in 10.000183 seconds, and the final result that led to the SERVFAIL was determined at line 2970 of source file resolver.c.

The next part shows the detected final result and the latest result of DNSSEC validation. The latter is always "success" when no validation attempt was made. In this example, this query probably resulted in SERVFAIL because all name servers are down or unreachable, leading to a timeout in 10 seconds. DNSSEC validation was probably not attempted.

The last part, enclosed in square brackets, shows statistics collected for this particular resolution attempt. The domain field shows the deepest zone that the resolver reached; it is the zone where the error was finally detected. The meaning of the other fields is summarized in the following table.

`referral`	The number of referrals the resolver received throughout the resolution process. In the above example there are two, which are most likely com and example.com.
`restart`	The number of cycles that the resolver tried remote servers at the `domain` zone. In each cycle, the resolver sends one query (possibly resending it, depending on the response) to each known name server of the `domain` zone.
`qrysent`	The number of queries the resolver sent at the `domain` zone.
`timeout`	The number of timeouts since the resolver received the last response.
`lame`	The number of lame servers the resolver detected at the `domain` zone. A server is detected to be lame either by an invalid response or as a result of lookup in BIND 9's address database (ADB), where lame servers are cached.
`quota`	The number of times the resolver was unable to send a query because it had exceeded the permissible fetch quota for a server.
`neterr`	The number of erroneous results that the resolver encountered in sending queries at the `domain` zone. One common case is when the remote server is unreachable and the resolver receives an "ICMP unreachable" error message.
`badresp`	The number of unexpected responses (other than `lame`) to queries sent by the resolver at the `domain` zone.
`adberr`	Failures in finding remote server addresses of the `domain` zone in the ADB. One common case of this is that the remote server's name does not have any address records.
`findfail`	Failures to resolve remote server addresses. This is a total number of failures throughout the resolution process.
`valfail`	Failures of DNSSEC validation. Validation failures are counted throughout the resolution process (not limited to the `domain` zone), but should only happen in `domain`.

At debug level 3 or higher, the same messages as those at debug level 1 are logged for errors other than SERVFAIL. Note that negative responses such as NXDOMAIN are not errors, and are not logged at this debug level.

At debug level 4 or higher, the detailed context information logged at debug level 2 is logged for errors other than SERVFAIL and for negative responses such as NXDOMAIN.

lwres Statement Grammar

This is the grammar of the lwres statement in the named.conf file:

lwres {
  [ listen-on {
    ( ip_addr [ port ip_port ] [ dscp ip_dscp ] ; )
      ...
    }; ]
  [ view view_name; ]
  [ search { domain_name ; ... }; ]
  [ ndots number; ]
  [ lwres-tasks number; ]
  [ lwres-clients number; ]
};

lwres Statement Definition and Usage

The lwres statement configures the name server to also act as a lightweight resolver server. (See the section called “Running a Resolver Daemon”.) There may be multiple lwres statements configuring lightweight resolver servers with different properties.

The listen-on statement specifies a list of IPv4 addresses (and ports) that this instance of a lightweight resolver daemon should accept requests on. If no port is specified, port 921 is used. If this statement is omitted, requests are accepted on 127.0.0.1, port 921.

The view statement binds this instance of a lightweight resolver daemon to a view in the DNS namespace, so that the response is constructed in the same manner as a normal DNS query matching this view. If this statement is omitted, the default view is used; if there is no default view, an error is triggered.

The search statement is equivalent to the search statement in /etc/resolv.conf. It provides a list of domains which are appended to relative names in queries.

The ndots statement is equivalent to the ndots statement in /etc/resolv.conf. It indicates the minimum number of dots in a relative domain name that should result in an exact-match lookup before search path elements are appended.

The lwres-tasks statement specifies the number of worker threads the lightweight resolver dedicates to serving clients. By default, the number is the same as the number of CPUs on the system; this can be overridden using the -n command-line option when starting the server.

The lwres-clients statement specifies the number of client objects per thread the lightweight resolver should create to serve client queries. By default, if the lightweight resolver runs as a part of named, 256 client objects are created for each task; if it runs as lwresd, 1024 client objects are created for each thread. The maximum value is 32768; higher values are silently ignored and the maximum is used instead. Note that setting too high a value may overconsume system resources.

The maximum number of client queries that the lightweight resolver can handle at any one time equals lwres-tasks times lwres-clients.

masters Statement Grammar

masters string [ port integer ] [ dscp
    integer ] { ( masters | ipv4_address [
    port integer ] | ipv6_address [ port
    integer ] ) [ key string ]; ... };

masters Statement Definition and Usage

masters lists allow for a common set of primaries to be easily used by multiple stub and secondary zones in their masters or also-notify lists.

options Statement Grammar

This is the grammar of the options statement in the named.conf file:

options {
	acache-cleaning-interval integer;
	acache-enable boolean;
	additional-from-auth boolean;
	additional-from-cache boolean;
	allow-new-zones boolean;
	allow-notify { address_match_element; ... };
	allow-query { address_match_element; ... };
	allow-query-cache { address_match_element; ... };
	allow-query-cache-on { address_match_element; ... };
	allow-query-on { address_match_element; ... };
	allow-recursion { address_match_element; ... };
	allow-recursion-on { address_match_element; ... };
	allow-transfer { address_match_element; ... };
	allow-update { address_match_element; ... };
	allow-update-forwarding { address_match_element; ... };
	also-notify [ port integer ] [ dscp integer ] { ( masters |
	    ipv4_address [ port integer ] | ipv6_address [ port
	    integer ] ) [ key string ]; ... };
	alt-transfer-source ( ipv4_address | * ) [ port ( integer | * )
	    ] [ dscp integer ];
	alt-transfer-source-v6 ( ipv6_address | * ) [ port ( integer |
	    * ) ] [ dscp integer ];
	answer-cookie boolean;
	attach-cache string;
	auth-nxdomain boolean; // default changed
	auto-dnssec ( allow | maintain | off );
	automatic-interface-scan boolean;
	avoid-v4-udp-ports { portrange; ... };
	avoid-v6-udp-ports { portrange; ... };
	bindkeys-file quoted_string;
	blackhole { address_match_element; ... };
	cache-file quoted_string;
	catalog-zones { zone string [ default-masters [ port integer ]
	    [ dscp integer ] { ( masters | ipv4_address [ port
	    integer ] | ipv6_address [ port integer ] ) [ key
	    string ]; ... } ] [ zone-directory quoted_string ] [
	    in-memory boolean ] [ min-update-interval integer ]; ... };
	check-dup-records ( fail | warn | ignore );
	check-integrity boolean;
	check-mx ( fail | warn | ignore );
	check-mx-cname ( fail | warn | ignore );
	check-names ( master | slave | response
	    ) ( fail | warn | ignore );
	check-sibling boolean;
	check-spf ( warn | ignore );
	check-srv-cname ( fail | warn | ignore );
	check-wildcard boolean;
	cleaning-interval integer;
	clients-per-query integer;
	cookie-algorithm ( aes | sha1 | sha256 | siphash24 );
	cookie-secret string;
	coresize ( default | unlimited | sizeval );
	datasize ( default | unlimited | sizeval );
	deny-answer-addresses { address_match_element; ... } [
	    except-from { quoted_string; ... } ];
	deny-answer-aliases { quoted_string; ... } [ except-from {
	    quoted_string; ... } ];
	dialup ( notify | notify-passive | passive | refresh | boolean );
	directory quoted_string;
	disable-algorithms string { string;
	    ... };
	disable-ds-digests string { string;
	    ... };
	disable-empty-zone string;
	dns64 netprefix {
		break-dnssec boolean;
		clients { address_match_element; ... };
		exclude { address_match_element; ... };
		mapped { address_match_element; ... };
		recursive-only boolean;
		suffix ipv6_address;
	};
	dns64-contact string;
	dns64-server string;
	dnssec-accept-expired boolean;
	dnssec-dnskey-kskonly boolean;
	dnssec-enable boolean;
	dnssec-loadkeys-interval integer;
	dnssec-lookaside ( string trust-anchor
	    string | auto | no );
	dnssec-must-be-secure string boolean;
	dnssec-secure-to-insecure boolean;
	dnssec-update-mode ( maintain | no-resign );
	dnssec-validation ( yes | no | auto );
	dnstap { ( all | auth | client | forwarder |
	    resolver ) [ ( query | response ) ]; ... };
	dnstap-identity ( quoted_string | none |
	    hostname );
	dnstap-output ( file | unix ) quoted_string;
	dnstap-version ( quoted_string | none );
	dscp integer;
	dual-stack-servers [ port integer ] { ( quoted_string [ port
	    integer ] [ dscp integer ] | ipv4_address [ port
	    integer ] [ dscp integer ] | ipv6_address [ port
	    integer ] [ dscp integer ] ); ... };
	dump-file quoted_string;
	edns-udp-size integer;
	empty-contact string;
	empty-server string;
	empty-zones-enable boolean;
	fetch-quota-params integer fixedpoint fixedpoint fixedpoint;
	fetches-per-server integer [ ( drop | fail ) ];
	fetches-per-zone integer [ ( drop | fail ) ];
	files ( default | unlimited | sizeval );
	filter-aaaa { address_match_element; ... };
	filter-aaaa-on-v4 ( break-dnssec | boolean );
	filter-aaaa-on-v6 ( break-dnssec | boolean );
	flush-zones-on-shutdown boolean;
	forward ( first | only );
	forwarders [ port integer ] [ dscp integer ] { ( ipv4_address
	    | ipv6_address ) [ port integer ] [ dscp integer ]; ... };
	fstrm-set-buffer-hint integer;
	fstrm-set-flush-timeout integer;
	fstrm-set-input-queue-size integer;
	fstrm-set-output-notify-threshold integer;
	fstrm-set-output-queue-model ( mpsc | spsc );
	fstrm-set-output-queue-size integer;
	fstrm-set-reopen-interval integer;
	geoip-directory ( quoted_string | none );
	geoip-use-ecs boolean;
	heartbeat-interval integer;
	hostname ( quoted_string | none );
	inline-signing boolean;
	interface-interval integer;
	ixfr-from-differences ( master | slave | boolean );
	keep-response-order { address_match_element; ... };
	key-directory quoted_string;
	lame-ttl ttlval;
	listen-on [ port integer ] [ dscp
	    integer ] {
	    address_match_element; ... };
	listen-on-v6 [ port integer ] [ dscp
	    integer ] {
	    address_match_element; ... };
	lmdb-mapsize sizeval;
	lock-file ( quoted_string | none );
	managed-keys-directory quoted_string;
	masterfile-format ( map | raw | text );
	masterfile-style ( full | relative );
	match-mapped-addresses boolean;
	max-acache-size ( unlimited | sizeval );
	max-cache-size ( default | unlimited | sizeval | percentage );
	max-cache-ttl integer;
	max-clients-per-query integer;
	max-journal-size ( unlimited | sizeval );
	max-ncache-ttl integer;
	max-records integer;
	max-recursion-depth integer;
	max-recursion-queries integer;
	max-refresh-time integer;
	max-retry-time integer;
	max-rsa-exponent-size integer;
	max-transfer-idle-in integer;
	max-transfer-idle-out integer;
	max-transfer-time-in integer;
	max-transfer-time-out integer;
	max-udp-size integer;
	max-zone-ttl ( unlimited | ttlval );
	memstatistics boolean;
	memstatistics-file quoted_string;
	message-compression boolean;
	min-refresh-time integer;
	min-retry-time integer;
	minimal-any boolean;
	minimal-responses ( no-auth | no-auth-recursive | boolean );
	multi-master boolean;
	no-case-compress { address_match_element; ... };
	nocookie-udp-size integer;
	notify ( explicit | master-only | boolean );
	notify-delay integer;
	notify-rate integer;
	notify-source ( ipv4_address | * ) [ port ( integer | * ) ] [
	    dscp integer ];
	notify-source-v6 ( ipv6_address | * ) [ port ( integer | * ) ]
	    [ dscp integer ];
	notify-to-soa boolean;
	nta-lifetime ttlval;
	nta-recheck ttlval;
	nxdomain-redirect string;
	pid-file ( quoted_string | none );
	port integer;
	preferred-glue string;
	prefetch integer [ integer ];
	provide-ixfr boolean;
	query-source ( ( [ address ] ( ipv4_address | * ) [ port (
	    integer | * ) ] ) | ( [ [ address ] ( ipv4_address | * ) ]
	    port ( integer | * ) ) ) [ dscp integer ];
	query-source-v6 ( ( [ address ] ( ipv6_address | * ) [ port (
	    integer | * ) ] ) | ( [ [ address ] ( ipv6_address | * ) ]
	    port ( integer | * ) ) ) [ dscp integer ];
	querylog boolean;
	random-device quoted_string;
	rate-limit {
		all-per-second integer;
		errors-per-second integer;
		exempt-clients { address_match_element; ... };
		ipv4-prefix-length integer;
		ipv6-prefix-length integer;
		log-only boolean;
		max-table-size integer;
		min-table-size integer;
		nodata-per-second integer;
		nxdomains-per-second integer;
		qps-scale integer;
		referrals-per-second integer;
		responses-per-second integer;
		slip integer;
		window integer;
	};
	recursing-file quoted_string;
	recursion boolean;
	recursive-clients integer;
	request-expire boolean;
	request-ixfr boolean;
	request-nsid boolean;
	require-server-cookie boolean;
	reserved-sockets integer;
	resolver-query-timeout integer;
	response-policy { zone string [ log boolean ] [ max-policy-ttl
	    integer ] [ policy ( cname | disabled | drop | given | no-op
	    | nodata | nxdomain | passthru | tcp-only quoted_string ) ] [
	    recursive-only boolean ]; ... } [ break-dnssec boolean ] [
	    max-policy-ttl integer ] [ min-ns-dots integer ] [
	    nsip-wait-recurse boolean ] [ qname-wait-recurse boolean ]
	    [ recursive-only boolean ];
	root-delegation-only [ exclude { quoted_string; ... } ];
	root-key-sentinel boolean;
	rrset-order { [ class string ] [ type string ] [ name
	    quoted_string ] string string; ... };
	secroots-file quoted_string;
	send-cookie boolean;
	serial-query-rate integer;
	serial-update-method ( date | increment | unixtime );
	server-id ( quoted_string | none | hostname );
	servfail-ttl ttlval;
	session-keyalg string;
	session-keyfile ( quoted_string | none );
	session-keyname string;
	sig-signing-nodes integer;
	sig-signing-signatures integer;
	sig-signing-type integer;
	sig-validity-interval integer [ integer ];
	sortlist { address_match_element; ... };
	stacksize ( default | unlimited | sizeval );
	startup-notify-rate integer;
	statistics-file quoted_string;
	tcp-clients integer;
	tcp-listen-queue integer;
	tkey-dhkey quoted_string integer;
	tkey-domain quoted_string;
	tkey-gssapi-credential quoted_string;
	tkey-gssapi-keytab quoted_string;
	transfer-format ( many-answers | one-answer );
	transfer-message-size integer;
	transfer-source ( ipv4_address | * ) [ port ( integer | * ) ] [
	    dscp integer ];
	transfer-source-v6 ( ipv6_address | * ) [ port ( integer | * )
	    ] [ dscp integer ];
	transfers-in integer;
	transfers-out integer;
	transfers-per-ns integer;
	trust-anchor-telemetry boolean; // experimental
	try-tcp-refresh boolean;
	update-check-ksk boolean;
	use-alt-transfer-source boolean;
	use-v4-udp-ports { portrange; ... };
	use-v6-udp-ports { portrange; ... };
	v6-bias integer;
	version ( quoted_string | none );
	zero-no-soa-ttl boolean;
	zero-no-soa-ttl-cache boolean;
	zone-statistics ( full | terse | none | boolean );
};

options Statement Definition and Usage

The options statement sets up global options to be used by BIND. This statement may appear only once in a configuration file. If there is no options statement, an options block with each option set to its default is used.

attach-cache

This option allows multiple views to share a single cache database. Each view has its own cache database by default, but if multiple views have the same operational policy for name resolution and caching, those views can share a single cache to save memory, and possibly improve resolution efficiency, by using this option.

The attach-cache option may also be specified in view statements, in which case it overrides the global attach-cache option.

The cache_name specifies the cache to be shared. When the named server configures views which are supposed to share a cache, it creates a cache with the specified name for the first view of these sharing views. The rest of the views simply refer to the already-created cache.

One common configuration to share a cache is to allow all views to share a single cache. This can be done by specifying attach-cache as a global option with an arbitrary name.

Another possible operation is to allow a subset of all views to share a cache while the others retain their own caches. For example, if there are three views A, B, and C, and only A and B should share a cache, specify the attach-cache option as a view of A (or B)'s option, referring to the other view name:

  view "A" {
    // this view has its own cache
    ...
  };
  view "B" {
    // this view refers to A's cache
    attach-cache "A";
  };
  view "C" {
    // this view has its own cache
    ...
  };

Views that share a cache must have the same policy on configurable parameters that may affect caching. The current implementation requires the following configurable options be consistent among these views: check-names, cleaning-interval, dnssec-accept-expired, dnssec-validation, max-cache-ttl, max-ncache-ttl, max-cache-size, and zero-no-soa-ttl.

Note that there may be other parameters that may cause confusion if they are inconsistent for different views that share a single cache. For example, if these views define different sets of forwarders that can return different answers for the same question, sharing the answer does not make sense or could even be harmful. It is administrator's responsibility to ensure that configuration differences in different views do not cause disruption with a shared cache.

directory

This sets the working directory of the server. Any non-absolute pathnames in the configuration file are taken as relative to this directory. The default location for most server output files (e.g., named.run) is this directory. If a directory is not specified, the working directory defaults to ".", the directory from which the server was started. The directory specified should be an absolute path. It is strongly recommended that the directory be writable by the effective user ID of the named process.

dnstap

dnstap is a fast, flexible method for capturing and logging DNS traffic. Developed by Robert Edmonds at Farsight Security, Inc., and supported by multiple DNS implementations, dnstap uses libfstrm (a lightweight high-speed framing library, see https://github.com/farsightsec/fstrm) to send event payloads which are encoded using Protocol Buffers (libprotobuf-c, a mechanism for serializing structured data developed by Google, Inc.; see https://developers.google.com/protocol-buffers).

To enable dnstap at compile time, the fstrm and protobuf-c libraries must be available, and BIND must be configured with --enable-dnstap.

The dnstap option is a bracketed list of message types to be logged. These may be set differently for each view. Supported types are client, auth, resolver, and forwarder. Specifying type all causes all dnstap messages to be logged, regardless of type.

Each type may take an additional argument to indicate whether to log query messages or response messages; if not specified, both queries and responses are logged.

Example: To log all authoritative queries and responses, recursive client responses, and upstream queries sent by the resolver, use:

dnstap {
  auth;
  client response;
  resolver query;
};

Logged dnstap messages can be parsed using the dnstap-read utility (see dnstap-read(1) for details).

For more information on dnstap, see http://dnstap.info.

The fstrm library has a number of tunables that are exposed in named.conf, and can be modified if necessary to improve performance or prevent loss of data. These are:

fstrm-set-buffer-hint: The threshold number of bytes to accumulate in the output buffer before forcing a buffer flush. The minimum is 1024, the maximum is 65536, and the default is 8192.
fstrm-set-flush-timeout: The number of seconds to allow unflushed data to remain in the output buffer. The minimum is 1 second, the maximum is 600 seconds (10 minutes), and the default is 1 second.
fstrm-set-output-notify-threshold: The number of outstanding queue entries to allow on an input queue before waking the I/O thread. The minimum is 1 and the default is 32.
fstrm-set-output-queue-model: The queuing semantics to use for queue objects. The default is mpsc (multiple producer, single consumer); the other option is spsc (single producer, single consumer).
fstrm-set-input-queue-size: The number of queue entries to allocate for each input queue. This value must be a power of 2. The minimum is 2, the maximum is 16384, and the default is 512.
fstrm-set-output-queue-size: The number of queue entries to allocate for each output queue. The minimum is 2, the maximum is system-dependent and based on IOV_MAX, and the default is 64.
fstrm-set-reopen-interval: The number of seconds to wait between attempts to reopen a closed output stream. The minimum is 1 second, the maximum is 600 seconds (10 minutes), and the default is 5 seconds.

Note that all of the above minimum, maximum, and default values are set by the libfstrm library, and may be subject to change in future versions of the library. See the libfstrm documentation for more information.

dnstap-output

This configures the path to which the dnstap frame stream is sent if dnstap is enabled at compile time and active.

The first argument is either file or unix, indicating whether the destination is a file or a Unix domain socket. The second argument is the path of the file or socket. (Note: when using a socket, dnstap messages are only sent if another process such as fstrm_capture (provided with libfstrm) is listening on the socket.)

dnstap-output can only be set globally in options. Currently, it can only be set once while named is running; once set, it cannot be changed by rndc reload or rndc reconfig.

dnstap-identity

This specifies an identity string to send in dnstap messages. If set to hostname, which is the default, the server's hostname is sent. If set to none, no identity string is sent.

dnstap-version

This specifies a version string to send in dnstap messages. The default is the version number of the BIND release. If set to none, no version string is sent.

geoip-directory

When named is compiled using the MaxMind GeoIP2 geolocation API, or the legacy GeoIP API, this specifies the directory containing GeoIP database files. By default, the option is set based on the prefix used to build the libmaxminddb module; for example, if the library is installed in /usr/local/lib, then the default geoip-directory is /usr/local/share/GeoIP. On Windows, the default is the named working directory. See the section called “acl Statement Definition and Usage” for details about geoip ACLs.

key-directory

This is the directory where the public and private DNSSEC key files should be found when performing a dynamic update of secure zones, if different than the current working directory. (Note that this option has no effect on the paths for files containing non-DNSSEC keys such as bind.keys, rndc.key, or session.key.)

lmdb-mapsize

When named is built with liblmdb, this option sets a maximum size for the memory map of the new-zone database (NZD) in LMDB database format. This database is used to store configuration information for zones added using rndc addzone. Note that this is not the NZD database file size, but the largest size that the database may grow to.

Because the database file is memory mapped, its size is limited by the address space of the named process. The default of 32 megabytes was chosen to be usable with 32-bit named builds. The largest permitted value is 1 terabyte. Given typical zone configurations without elaborate ACLs, a 32 MB NZD file ought to be able to hold configurations of about 100,000 zones.

managed-keys-directory

This specifies the directory in which to store the files that track managed DNSSEC keys. By default, this is the working directory. The directory must be writable by the effective user ID of the named process.

If named is not configured to use views, managed keys for the server are tracked in a single file called managed-keys.bind. Otherwise, managed keys are tracked in separate files, one file per view; each file name is the view name (or, if it contains characters that are incompatible with use as a file name, the SHA256 hash of the view name), followed by the extension .mkeys.

(Note: in earlier releases, file names for views always used the SHA256 hash of the view name. To ensure compatibility after upgrading, if a file using the old name format is found to exist, it is used instead of the new format.)

named-xfer

This option is obsolete. In BIND 9, no separate named-xfer program is needed; its functionality is built into the name server.

tkey-gssapi-keytab

This is the KRB5 keytab file to use for GSS-TSIG updates. If this option is set and tkey-gssapi-credential is not set, updates are allowed with any key matching a principal in the specified keytab.

tkey-gssapi-credential

This is the security credential with which the server should authenticate keys requested by the GSS-TSIG protocol. Currently only Kerberos 5 authentication is available; the credential is a Kerberos principal which the server can acquire through the default system key file, normally /etc/krb5.keytab. The location of the keytab file can be overridden using the tkey-gssapi-keytab option. Normally this principal is of the form "DNS/server.domain". To use GSS-TSIG, tkey-domain must also be set if a specific keytab is not set with tkey-gssapi-keytab.

tkey-domain

This domain is appended to the names of all shared keys generated with TKEY. When a client requests a TKEY exchange, it may or may not specify the desired name for the key. If present, the name of the shared key is client-specified part + tkey-domain. Otherwise, the name of the shared key is random hex digits + tkey-domain. In most cases, the domainname should be the server's domain name, or an otherwise nonexistent subdomain like "_tkey.domainname". If using GSS-TSIG, this variable must be defined, unless a specific keytab is specified using tkey-gssapi-keytab.

tkey-dhkey

This is the Diffie-Hellman key used by the server to generate shared keys with clients using the Diffie-Hellman mode of TKEY. The server must be able to load the public and private keys from files in the working directory. In most cases, the key_name should be the server's host name.

cache-file

This is for testing only. Do not use.

dump-file

This is the pathname of the file the server dumps the database to, when instructed to do so with rndc dumpdb. If not specified, the default is named_dump.db.

memstatistics-file

This is the pathname of the file the server writes memory usage statistics to on exit. If not specified, the default is named.memstats.

lock-file

This is the pathname of a file on which named attempts to acquire a file lock when starting for the first time; if unsuccessful, the server terminates, under the assumption that another server is already running. If not specified, the default is none.

Specifying lock-file none disables the use of a lock file. lock-file is ignored if named was run using the -X option, which overrides it. Changes to lock-file are ignored if named is being reloaded or reconfigured; it is only effective when the server is first started.

pid-file

This is the pathname of the file the server writes its process ID in. If not specified, the default is /var/run/named/named.pid. The PID file is used by programs that send signals to the running name server. Specifying pid-file none disables the use of a PID file; no file is written and any existing one is removed. Note that none is a keyword, not a filename, and therefore is not enclosed in double quotes.

recursing-file

This is the pathname of the file where the server dumps the queries that are currently recursing, when instructed to do so with rndc recursing. If not specified, the default is named.recursing.

statistics-file

This is the pathname of the file the server appends statistics to, when instructed to do so using rndc stats. If not specified, the default is named.stats in the server's current directory. The format of the file is described in the section called “The Statistics File”.

bindkeys-file

This is the pathname of a file to override the built-in trusted keys provided by named. See the discussion of dnssec-validation for details. If not specified, the default is /etc/bind.keys.

secroots-file

This is the pathname of the file the server dumps security roots to, when instructed to do so with rndc secroots. If not specified, the default is named.secroots.

session-keyfile

This is the pathname of the file into which to write a TSIG session key generated by named for use by nsupdate -l. If not specified, the default is /var/run/named/session.key. (See the section called “Dynamic Update Policies”, and in particular the discussion of the update-policy statement's local option for more information about this feature.)

session-keyname

This is the key name to use for the TSIG session key. If not specified, the default is local-ddns.

session-keyalg

This is the algorithm to use for the TSIG session key. Valid values are hmac-sha1, hmac-sha224, hmac-sha256, hmac-sha384, hmac-sha512, and hmac-md5. If not specified, the default is hmac-sha256.

port

This is the UDP/TCP port number the server uses to receive and send DNS protocol traffic. The default is 53. This option is mainly intended for server testing; a server using a port other than 53 is not able to communicate with the global DNS.

dscp

This is the global Differentiated Services Code Point (DSCP) value to classify outgoing DNS traffic, on operating systems that support DSCP. Valid values are 0 through 63. It is not configured by default.

random-device

This specifies a source of entropy to be used by the server. Entropy is primarily needed for DNSSEC operations, such as TKEY transactions and dynamic update of signed zones. This option specifies the device (or file) from which to read entropy. If it is a file, operations requiring entropy will fail when the file has been exhausted. If random-device is not specified, the default value is /dev/random (or equivalent) when present, and none otherwise. The random-device option takes effect during the initial configuration load at server startup time and is ignored on subsequent reloads.

preferred-glue

If specified, the listed type (A or AAAA) is emitted before other glue in the additional section of a query response. The default is to prefer A records when responding to queries that arrived via IPv4 and AAAA when responding to queries that arrived via IPv6.

root-delegation-only

This turns on enforcement of delegation-only in TLDs (top-level domains) and root zones with an optional exclude list.

DS queries are expected to be made to and be answered by delegation-only zones. Such queries and responses are treated as an exception to delegation-only processing and are not converted to NXDOMAIN responses, provided a CNAME is not discovered at the query name.

If a delegation-only zone server also serves a child zone, it is not always possible to determine whether an answer comes from the delegation-only zone or the child zone. SOA NS and DNSKEY records are apex-only records and a matching response that contains these records or DS is treated as coming from a child zone. RRSIG records are also examined to see if they are signed by a child zone, and the authority section is examined to see if there is evidence that the answer is from the child zone. Answers that are determined to be from a child zone are not converted to NXDOMAIN responses. Despite all these checks, there is still a possibility of false negatives when a child zone is being served.

Similarly, false positives can arise from empty nodes (no records at the name) in the delegation-only zone when the query type is not ANY.

Note that some TLDs are not delegation-only; e.g., "DE", "LV", "US", and "MUSEUM". This list is not exhaustive.

options {
        root-delegation-only exclude { "de"; "lv"; "us"; "museum"; };
};

disable-algorithms

This disables the specified DNSSEC algorithms at and below the specified name. Multiple disable-algorithms statements are allowed. Only the best-match disable-algorithms clause is used to determine the algorithms.

If all supported algorithms are disabled, the zones covered by the disable-algorithms setting are treated as insecure.

Configured trust anchors in trusted-keys or managed-keys that match a disabled algorithm are ignored and treated as if they were not configured.

disable-ds-digests

This disables the specified DS digest types at and below the specified name. Multiple disable-ds-digests statements are allowed. Only the best-match disable-ds-digests clause is used to determine the digest types.

If all supported digest types are disabled, the zones covered by disable-ds-digests are treated as insecure.

dnssec-lookaside

When set, dnssec-lookaside provides the validator with an alternate method to validate DNSKEY records at the top of a zone. When a DNSKEY is at or below a domain specified by the deepest dnssec-lookaside, and the normal DNSSEC validation has left the key untrusted, the trust-anchor is appended to the key name and a DLV record is looked up to see if it can validate the key. If the DLV record validates a DNSKEY (similarly to the way a DS record does), the DNSKEY RRset is deemed to be trusted.

If dnssec-lookaside is set to no, then dnssec-lookaside is not used.

Note: the ISC-provided DLV service at dlv.isc.org has been shut down. The dnssec-lookaside auto; configuration option, which set named to use ISC DLV with minimal configuration, has accordingly been removed.

dnssec-must-be-secure

This specifies hierarchies which must be or may not be secure (signed and validated). If yes, then named only accepts answers if they are secure. If no, then normal DNSSEC validation applies, allowing insecure answers to be accepted. The specified domain must be under a trusted-keys or managed-keys statement, or dnssec-validation auto must be active.

dns64

This directive instructs named to return mapped IPv4 addresses to AAAA queries when there are no AAAA records. It is intended to be used in conjunction with a NAT64. Each dns64 defines one DNS64 prefix. Multiple DNS64 prefixes can be defined.

Compatible IPv6 prefixes have lengths of 32, 40, 48, 56, 64, and 96, per RFC 6052. Bits 64..71 inclusive must be zero, with the most significant bit of the prefix in position 0.

In addition, a reverse IP6.ARPA zone is created for the prefix to provide a mapping from the IP6.ARPA names to the corresponding IN-ADDR.ARPA names using synthesized CNAMEs. dns64-server and dns64-contact can be used to specify the name of the server and contact for the zones. These can be set at the view/options level but not on a per-prefix basis.

Each dns64 supports an optional clients ACL that determines which clients are affected by this directive. If not defined, it defaults to any;.

Each dns64 supports an optional mapped ACL that selects which IPv4 addresses are to be mapped in the corresponding A RRset. If not defined, it defaults to any;.

Normally, DNS64 does not apply to a domain name that owns one or more AAAA records; these records are simply returned. The optional exclude ACL allows specification of a list of IPv6 addresses that are ignored if they appear in a domain name's AAAA records; DNS64 is applied to any A records the domain name owns. If not defined, exclude defaults to ::ffff:0.0.0.0/96.

A optional suffix can also be defined to set the bits trailing the mapped IPv4 address bits. By default these bits are set to ::. The bits matching the prefix and mapped IPv4 address must be zero.

If recursive-only is set to yes, the DNS64 synthesis only happens for recursive queries. The default is no.

If break-dnssec is set to yes, the DNS64 synthesis happens even if the result, if validated, would cause a DNSSEC validation failure. If this option is set to no (the default), the DO is set on the incoming query, and there are RRSIGs on the applicable records, then synthesis does not happen.

        acl rfc1918 { 10/8; 192.168/16; 172.16/12; };

        dns64 64:FF9B::/96 {
                clients { any; };
                mapped { !rfc1918; any; };
                exclude { 64:FF9B::/96; ::ffff:0000:0000/96; };
                suffix ::;
        };

dnssec-loadkeys-interval

When a zone is configured with auto-dnssec maintain;, its key repository must be checked periodically to see if any new keys have been added or any existing keys' timing metadata has been updated (see dnssec-keygen(8) and dnssec-settime(8)). The dnssec-loadkeys-interval option sets the frequency of automatic repository checks, in minutes. The default is 60 (1 hour), the minimum is 1 (1 minute), and the maximum is 1440 (24 hours); any higher value is silently reduced.

dnssec-update-mode

If this option is set to its default value of maintain in a zone of type master which is DNSSEC-signed and configured to allow dynamic updates (see the section called “Dynamic Update Policies”), and if named has access to the private signing key(s) for the zone, then named automatically signs all new or changed records and maintains signatures for the zone by regenerating RRSIG records whenever they approach their expiration date.

If the option is changed to no-resign, then named signs all new or changed records, but scheduled maintenance of signatures is disabled.

With either of these settings, named rejects updates to a DNSSEC-signed zone when the signing keys are inactive or unavailable to named. (A planned third option, external, will disable all automatic signing and allow DNSSEC data to be submitted into a zone via dynamic update; this is not yet implemented.)

nta-lifetime

This specifies the default lifetime, in seconds, for negative trust anchors added via rndc nta.

A negative trust anchor selectively disables DNSSEC validation for zones that are known to be failing because of misconfiguration, rather than an attack. When data to be validated is at or below an active NTA (and above any other configured trust anchors), named aborts the DNSSEC validation process and treats the data as insecure rather than bogus. This continues until the NTA's lifetime is elapsed. NTAs persist across named restarts.

For convenience, TTL-style time-unit suffixes can be used to specify the NTA lifetime in seconds, minutes, or hours. nta-lifetime defaults to one hour; it cannot exceed one week.

nta-recheck

This specifies how often to check whether negative trust anchors added via rndc nta are still necessary.

A negative trust anchor is normally used when a domain has stopped validating due to operator error; it temporarily disables DNSSEC validation for that domain. In the interest of ensuring that DNSSEC validation is turned back on as soon as possible, named periodically sends a query to the domain, ignoring negative trust anchors, to find out whether it can now be validated. If so, the negative trust anchor is allowed to expire early.

Validity checks can be disabled for an individual NTA by using rndc nta -f, or for all NTAs by setting nta-recheck to zero.

For convenience, TTL-style time-unit suffixes can be used to specify the NTA recheck interval in seconds, minutes, or hours. The default is five minutes. It cannot be longer than nta-lifetime, which cannot be longer than a week.

max-zone-ttl

This specifies a maximum permissible TTL value in seconds. For convenience, TTL-style time-unit suffixes may be used to specify the maximum value. When loading a zone file using a masterfile-format of text or raw, any record encountered with a TTL higher than max-zone-ttl causes the zone to be rejected.

This is useful in DNSSEC-signed zones because when rolling to a new DNSKEY, the old key needs to remain available until RRSIG records have expired from caches. The max-zone-ttl option guarantees that the largest TTL in the zone is no higher than the set value.

(Note: because map-format files load directly into memory, this option cannot be used with them.)

The default value is unlimited. A max-zone-ttl of zero is treated as unlimited.

serial-update-method

Zones configured for dynamic DNS may use this option to set the update method to be used for the zone serial number in the SOA record.

With the default setting of serial-update-method increment;, the SOA serial number is incremented by one each time the zone is updated.

When set to serial-update-method unixtime;, the SOA serial number is set to the number of seconds since the Unix epoch, unless the serial number is already greater than or equal to that value, in which case it is simply incremented by one.

When set to serial-update-method date;, the new SOA serial number is the current date in the form "YYYYMMDD", followed by two zeroes, unless the existing serial number is already greater than or equal to that value, in which case it is incremented by one.

zone-statistics

If full, the server collects statistical data on all zones, unless specifically turned off on a per-zone basis by specifying zone-statistics terse or zone-statistics none in the zone statement. The default is terse, providing minimal statistics on zones (including name and current serial number, but not query type counters).

These statistics may be accessed via the statistics-channel or using rndc stats, which dumps them to the file listed in the statistics-file. See also the section called “The Statistics File”.

For backward compatibility with earlier versions of BIND 9, the zone-statistics option can also accept yes or no; yes has the same meaning as full. As of BIND 9.10, no has the same meaning as none; previously, it was the same as terse.

Boolean Options

automatic-interface-scan

If yes and supported by the operating system, this automatically rescans network interfaces when the interface addresses are added or removed. The default is yes. This configuration option does not affect the time-based interface-interval option; it is recommended to set the time-based interface-interval to 0 when the operator confirms that automatic interface scanning is supported by the operating system.

The automatic-interface-scan implementation uses routing sockets for the network interface discovery; therefore, the operating system must support the routing sockets for this feature to work.

allow-new-zones

If yes, then zones can be added at runtime via rndc addzone. The default is no.

Newly added zones' configuration parameters are stored so that they can persist after the server is restarted. The configuration information is saved in a file called viewname.nzf (or, if named is compiled with liblmdb, in an LMDB database file called viewname.nzd). viewname is the name of the view, unless the view name contains characters that are incompatible with use as a file name, in which case a cryptographic hash of the view name is used instead.

Configurations for zones added at runtime are stored either in a new-zone file (NZF) or a new-zone database (NZD), depending on whether named was linked with liblmdb at compile time. See rndc(8) for further details about rndc addzone.

auth-nxdomain

If yes, then the AA bit is always set on NXDOMAIN responses, even if the server is not actually authoritative. The default is no.

deallocate-on-exit

This option was used in BIND 8 to enable checking for memory leaks on exit. BIND 9 ignores the option and always performs the checks.

memstatistics

This writes memory statistics to the file specified by memstatistics-file at exit. The default is no unless -m record is specified on the command line, in which case it is yes.

dialup

If yes, then the server treats all zones as if they are doing zone transfers across a dial-on-demand dialup link, which can be brought up by traffic originating from this server. Although this setting has different effects according to zone type, it concentrates the zone maintenance so that everything happens quickly, once every heartbeat-interval, ideally during a single call. It also suppresses some normal zone maintenance traffic. The default is no.

If specified in the view and zone statements, the dialup option overrides the global dialup option.

If the zone is a primary zone, the server sends out a NOTIFY request to all the secondaries (default). This should trigger the zone serial number check in the secondary (providing it supports NOTIFY), allowing the secondary to verify the zone while the connection is active. The set of servers to which NOTIFY is sent can be controlled by notify and also-notify.

If the zone is a secondary or stub zone, the server suppresses the regular "zone up to date" (refresh) queries and only performs them when the heartbeat-interval expires, in addition to sending NOTIFY requests.

Finer control can be achieved by using notify, which only sends NOTIFY messages; notify-passive, which sends NOTIFY messages and suppresses the normal refresh queries; refresh, which suppresses normal refresh processing and sends refresh queries when the heartbeat-interval expires; and passive, which disables normal refresh processing.

dialup mode	normal refresh	heart-beat refresh	heart-beat notify
no (default)	yes	no	no
yes	no	yes	yes
notify	yes	no	yes
refresh	no	yes	no
passive	no	no	no
notify-passive	no	no	yes

Note that normal NOTIFY processing is not affected by dialup.

fake-iquery

In BIND 8, this option enabled simulating the obsolete DNS query type IQUERY. BIND 9 never does IQUERY simulation.

fetch-glue

This option is obsolete. In BIND 8, fetch-glue yes caused the server to attempt to fetch glue resource records it did not have when constructing the additional data section of a response. This is now considered a bad idea and BIND 9 never does it.

flush-zones-on-shutdown

When the nameserver exits upon receiving SIGTERM, flush or do not flush any pending zone writes. The default is flush-zones-on-shutdown no.

geoip-use-ecs

When BIND is compiled with GeoIP support and configured with "geoip" ACL elements, this option indicates whether the EDNS Client Subnet option, if present in a request, should be used for matching against the GeoIP database. The default is geoip-use-ecs yes.

has-old-clients

This option was incorrectly implemented in BIND 8, and is ignored by BIND 9. To achieve the intended effect of has-old-clients yes, specify the two separate options auth-nxdomain yes and rfc2308-type1 no instead.

host-statistics

In BIND 8, this enabled keeping of statistics for every host that the name server interacts with. It is not implemented in BIND 9.

root-key-sentinel

If yes, respond to root key sentinel probes as described in draft-ietf-dnsop-kskroll-sentinel-08. The default is yes.

maintain-ixfr-base

This option is obsolete. It was used in BIND 8 to determine whether a transaction log was kept for Incremental Zone Transfer. BIND 9 maintains a transaction log whenever possible. To disable outgoing incremental zone transfers, use provide-ixfr no.

message-compression

If yes, DNS name compression is used in responses to regular queries (not including AXFR or IXFR, which always use compression). Setting this option to no reduces CPU usage on servers and may improve throughput. However, it increases response size, which may cause more queries to be processed using TCP; a server with compression disabled is out of compliance with RFC 1123 Section 6.1.3.2. The default is yes.

minimal-responses

If set to yes, then when generating responses the server only adds records to the authority and additional data sections when they are required (e.g. delegations, negative responses). This may improve the performance of the server.

When set to no-auth, the server omits records from the authority section unless they are required, but it may still add records to the additional section. When set to no-auth-recursive, this is only done if the query is recursive. These settings are useful when answering stub clients, which usually ignore the authority section. no-auth-recursive is designed for mixed-mode servers that handle both authoritative and recursive queries.

The default is no.

minimal-any

If set to yes, the server replies with only one of the RRsets for the query name, and its covering RRSIGs if any, when generating a positive response to a query of type ANY over UDP, instead of replying with all known RRsets for the name. Similarly, a query for type RRSIG is answered with the RRSIG records covering only one type. This can reduce the impact of some kinds of attack traffic, without harming legitimate clients. (Note, however, that the RRset returned is the first one found in the database; it is not necessarily the smallest available RRset.) Additionally, minimal-responses is turned on for these queries, so no unnecessary records are added to the authority or additional sections. The default is no.

multiple-cnames

This option was used in BIND 8 to allow a domain name to have multiple CNAME records, in violation of the DNS standards. BIND 9.2 onwards always strictly enforces the CNAME rules both in primary files and dynamic updates.

notify

If yes (the default), DNS NOTIFY messages are sent when a zone the server is authoritative for changes; see the section called “Notify”. The messages are sent to the servers listed in the zone's NS records (except the primary server identified in the SOA MNAME field), and to any servers listed in the also-notify option.

If master-only, notifies are only sent for primary zones. If explicit, notifies are sent only to servers explicitly listed using also-notify. If no, no notifies are sent.

The notify option may also be specified in the zone statement, in which case it overrides the options notify statement. It would only be necessary to turn off this option if it caused secondary zones to crash.

notify-to-soa

If yes, do not check the name servers in the NS RRset against the SOA MNAME. Normally a NOTIFY message is not sent to the SOA MNAME (SOA ORIGIN), as it is supposed to contain the name of the ultimate primary server. Sometimes, however, a secondary server is listed as the SOA MNAME in hidden primary configurations; in that case, the ultimate primary should be set to still send NOTIFY messages to all the name servers listed in the NS RRset.

recursion

If yes, and a DNS query requests recursion, then the server attempts to do all the work required to answer the query. If recursion is off and the server does not already know the answer, it returns a referral response. The default is yes. Note that setting recursion no does not prevent clients from getting data from the server's cache; it only prevents new data from being cached as an effect of client queries. Caching may still occur as an effect the server's internal operation, such as NOTIFY address lookups.

request-nsid

If yes, then an empty EDNS(0) NSID (Name Server Identifier) option is sent with all queries to authoritative name servers during iterative resolution. If the authoritative server returns an NSID option in its response, then its contents are logged in the resolver category at level info. The default is no.

request-sit

This experimental option is obsolete.

require-server-cookie

If yes, require a valid server cookie before sending a full response to a UDP request from a cookie-aware client. BADCOOKIE is sent if there is a bad or nonexistent server cookie. The default is no.

Users wishing to test that DNS COOKIE clients correctly handle BADCOOKIE, or who are getting a lot of forged DNS requests with DNS COOKIES present, should set this to yes. Setting this to yes results in a reduced amplification effect in a reflection attack, as the BADCOOKIE response is smaller than a full response, while also requiring a legitimate client to follow up with a second query with the new, valid, cookie.

answer-cookie

When set to the default value of yes, COOKIE EDNS options are sent when applicable in replies to client queries. If set to no, COOKIE EDNS options are not sent in replies. This can only be set at the global options level, not per-view.

answer-cookie no is only intended as a temporary measure, for use when named shares an IP address with other servers that do not yet support DNS COOKIE. A mismatch between servers on the same address is not expected to cause operational problems, but the option to disable COOKIE responses so that all servers have the same behavior is provided out of an abundance of caution. DNS COOKIE is an important security mechanism, and should not be disabled unless absolutely necessary.

send-cookie

If yes, then a COOKIE EDNS option is sent along with the query. If the resolver has previously communicated with the server, the COOKIE returned in the previous transaction is sent. This is used by the server to determine whether the resolver has talked to it before. A resolver sending the correct COOKIE is assumed not to be an off-path attacker sending a spoofed-source query; the query is therefore unlikely to be part of a reflection/amplification attack, so resolvers sending a correct COOKIE option are not subject to response rate limiting (RRL). Resolvers which do not send a correct COOKIE option may be limited to receiving smaller responses via the nocookie-udp-size option. The default is yes.

nocookie-udp-size

This sets the maximum size of UDP responses that are sent to queries without a valid server COOKIE. A value below 128 is silently raised to 128. The default value is 4096, but the max-udp-size option may further limit the response size as the default for max-udp-size is 1232.

sit-secret

This experimental option is obsolete.

cookie-algorithm

This sets the algorithm to be used when generating the server cookie; the options are "aes", "sha1", or "sha256". The default is "aes" if supported by the cryptographic library; otherwise, "sha256".

cookie-secret

If set, this is a shared secret used for generating and verifying EDNS COOKIE options within an anycast cluster. If not set, the system generates a random secret at startup. The shared secret is encoded as a hex string and needs to be 128 bits for AES128, 160 bits for SHA1, and 256 bits for SHA256.

If there are multiple secrets specified, the first one listed in named.conf is used to generate new server cookies. The others are only used to verify returned cookies.

rfc2308-type1

Setting this to yes causes the server to send NS records along with the SOA record for negative answers. The default is no.

Note

This is not yet implemented in BIND 9.

trust-anchor-telemetry

This causes named to send specially formed queries once per day to domains for which trust anchors have been configured via trusted-keys, managed-keys, or dnssec-validation auto.

The query name used for these queries has the form "_ta-xxxx(-xxxx)(...)".<domain>, where each "xxxx" is a group of four hexadecimal digits representing the key ID of a trusted DNSSEC key. The key IDs for each domain are sorted smallest to largest prior to encoding. The query type is NULL.

By monitoring these queries, zone operators are able to see which resolvers have been updated to trust a new key; this may help them decide when it is safe to remove an old one.

The default is yes.

use-id-pool

This option is obsolete. BIND 9 always allocates query IDs from a pool.

use-ixfr

This option is obsolete. To disable IXFR to a particular server or servers, see the information on the provide-ixfr option in the section called “server Statement Definition and Usage”. See also the section called “Incremental Zone Transfers (IXFR)”.

provide-ixfr

See the description of provide-ixfr in the section called “server Statement Definition and Usage”.

request-ixfr

See the description of request-ixfr in the section called “server Statement Definition and Usage”.

request-expire

See the description of request-expire in the section called “server Statement Definition and Usage”.

treat-cr-as-space

This option was used in BIND 8 to make the server treat carriage return ("\r") characters the same way as a space or tab character, to facilitate loading of zone files on a Unix system that were generated on an NT or DOS machine. In BIND 9, both UNIX "\n" and NT/DOS "\r\n" newlines are always accepted, and the option is ignored.

additional-from-auth, additional-from-cache

These options control the behavior of an authoritative server when answering queries which have additional data, or when following CNAME and DNAME chains.

When both of these options are set to yes (the default) and a query is being answered from authoritative data (a zone configured into the server), the additional data section of the reply is filled in using data from other authoritative zones and from the cache. In some situations this is undesirable, such as when there is concern over the correctness of the cache, or in servers where secondary zones may be added and modified by untrusted third parties. Also, avoiding the search for this additional data speeds up server operations at the possible expense of additional queries to resolve what would otherwise be provided in the additional section.

For example, if a query asks for an MX record for host foo.example.com, and the record found is "MX 10 mail.example.net", normally the address records (A and AAAA) for mail.example.net are provided as well, if known, even though they are not in the example.com zone. Setting these options to no disables this behavior and makes the server only search for additional data in the zone it answers from.

These options are intended for use in authoritative-only servers, or in authoritative-only views. Attempts to set them to no without also specifying recursion no will cause the server to ignore the options and log a warning message.

Specifying additional-from-cache no actually disables the use of the cache not only for additional data lookups but also when looking up the answer. This is usually the desired behavior in an authoritative-only server where the correctness of the cached data is an issue.

When a name server is non-recursively queried for a name that is not below the apex of any served zone, it normally answers with an "upwards referral" to the root servers or the servers of some other known parent of the query name. Since the data in an upwards referral comes from the cache, the server is not able to provide upwards referrals when additional-from-cache no has been specified. Instead, it responds to such queries with REFUSED. This should not cause any problems since upwards referrals are not required for the resolution process.

match-mapped-addresses

If yes, then an IPv4-mapped IPv6 address matches any address-match list entries that match the corresponding IPv4 address.

This option was introduced to work around a kernel quirk in some operating systems that causes IPv4 TCP connections, such as zone transfers, to be accepted on an IPv6 socket using mapped addresses. This caused address-match lists designed for IPv4 to fail to match. However, named now solves this problem internally. The use of this option is discouraged.

filter-aaaa-on-v4

This option is only available when BIND 9 is compiled with the --enable-filter-aaaa option on the "configure" command line. It is intended to help the transition from IPv4 to IPv6 by not giving IPv6 addresses to DNS clients unless they have connections to the IPv6 Internet. This is not recommended unless absolutely necessary. The default is no. The filter-aaaa-on-v4 option may also be specified in view statements to override the global filter-aaaa-on-v4 option.

If yes, the DNS client is at an IPv4 address, in filter-aaaa, and if the response does not include DNSSEC signatures, then all AAAA records are deleted from the response. This filtering applies to all responses and not only authoritative responses.

If break-dnssec, then AAAA records are deleted even when DNSSEC is enabled. As suggested by the name, this causes the response to not verify, because the DNSSEC protocol is designed to detect deletions.

This mechanism can erroneously cause other servers to not give AAAA records to their clients. A recursing server with both IPv6 and IPv4 network connections, that queries an authoritative server using this mechanism via IPv4, is denied AAAA records even if its client is using IPv6.

This mechanism is applied to authoritative as well as non-authoritative records. A client using IPv4 that is not allowed recursion can erroneously be given AAAA records because the server is not allowed to check for A records.

Some AAAA records are given to IPv4 clients in glue records. IPv4 clients that are servers can then erroneously answer requests for AAAA records received via IPv4.

filter-aaaa-on-v6

This is identical to filter-aaaa-on-v4, except it filters AAAA responses to queries from IPv6 clients instead of IPv4 clients. To filter all responses, set both options to yes.

ixfr-from-differences

When yes and the server loads a new version of a primary zone from its zone file or receives a new version of a secondary file via zone transfer, it compares the new version to the previous one and calculates a set of differences. The differences are then logged in the zone's journal file so that the changes can be transmitted to downstream secondaries as an incremental zone transfer.

By allowing incremental zone transfers to be used for non-dynamic zones, this option saves bandwidth at the expense of increased CPU and memory consumption at the primary server. In particular, if the new version of a zone is completely different from the previous one, the set of differences is of a size comparable to the combined size of the old and new zone versions, and the server needs to temporarily allocate memory to hold this complete difference set.

ixfr-from-differences also accepts master and slave at the view and options levels, which causes ixfr-from-differences to be enabled for all primary or secondary zones, respectively. It is off by default.

Note: if inline signing is enabled for a zone, the user-provided ixfr-from-differences setting is ignored for that zone.

multi-master

This should be set when there are multiple primary servers for a zone and the addresses refer to different machines. If yes, named does not log when the serial number on the primary is less than what named currently has. The default is no.

auto-dnssec

Zones configured for dynamic DNS may use this option to allow varying levels of automatic DNSSEC key management. There are three possible settings:

auto-dnssec allow; permits keys to be updated and the zone fully re-signed whenever the user issues the command rndc sign zonename.

auto-dnssec maintain; includes the above, but also automatically adjusts the zone's DNSSEC keys on a schedule, according to the keys' timing metadata (see dnssec-keygen(8) and dnssec-settime(8)). The command rndc sign zonename causes named to load keys from the key repository and sign the zone with all keys that are active. rndc loadkeys zonename causes named to load keys from the key repository and schedule key maintenance events to occur in the future, but it does not sign the full zone immediately. Note: once keys have been loaded for a zone the first time, the repository is searched for changes periodically, regardless of whether rndc loadkeys is used. The recheck interval is defined by dnssec-loadkeys-interval.)

The default setting is auto-dnssec off.

dnssec-enable

This indicates whether DNSSEC-related resource records are to be returned by named. If set to no, named does not return DNSSEC-related resource records unless specifically queried for. The default is yes.

dnssec-validation

This option enables DNSSEC validation in named. Note that dnssec-enable also needs to be set to yes to be effective. If set to no, DNSSEC validation is disabled.

If set to auto, DNSSEC validation is enabled and a default trust anchor for the DNS root zone is used. If set to yes, DNSSEC validation is enabled, but a trust anchor must be manually configured using a trusted-keys or managed-keys statement. The default is yes.

The default root trust anchor is stored in the file bind.keys. named loads that key at startup if dnssec-validation is set to auto. A copy of the file is installed along with BIND 9, and is current as of the release date. If the root key expires, a new copy of bind.keys can be downloaded from https://www.isc.org/bind-keys.

(To prevent problems if bind.keys is not found, the current trust anchor is also compiled in to named. Relying on this is not recommended, however, as it requires named to be recompiled with a new key when the root key expires.)

Note

named loads only the root key from bind.keys. The file cannot be used to store keys for other zones. The root key in bind.keys is ignored if dnssec-validation auto is not in use.

Whenever the resolver sends out queries to an EDNS-compliant server, it always sets the DO bit indicating it can support DNSSEC responses, even if dnssec-validation is off.

dnssec-accept-expired

This accepts expired signatures when verifying DNSSEC signatures. The default is no. Setting this option to yes leaves named vulnerable to replay attacks.

querylog

Query logging provides a complete log of all incoming queries and all query errors. This provides more insight into the server's activity, but with a cost to performance which may be significant on heavily loaded servers.

The querylog option specifies whether query logging should be active when named first starts. If querylog is not specified, then query logging is determined by the presence of the logging category queries. Query logging can also be activated at runtime using the command rndc querylog on, or deactivated with rndc querylog off.

check-names

This option is used to restrict the character set and syntax of certain domain names in zone files and/or DNS responses received from the network. The default varies according to usage area. For primary zones (i.e., type master), the default is fail. For secondary zones (type slave), the default is warn. For answers received from the network (response), the default is ignore.

The rules for legal hostnames and mail domains are derived from RFC 952 and RFC 821 as modified by RFC 1123.

check-names applies to the owner names of A, AAAA, and MX records. It also applies to the domain names in the RDATA of NS, SOA, MX, and SRV records. It further applies to the RDATA of PTR records where the owner name indicates that it is a reverse lookup of a hostname (the owner name ends in IN-ADDR.ARPA, IP6.ARPA, or IP6.INT).

check-dup-records

This checks primary zones for records that are treated as different by DNSSEC but are semantically equal in plain DNS. The default is to warn. Other possible values are fail and ignore.

check-mx

This checks whether the MX record appears to refer to a IP address. The default is to warn. Other possible values are fail and ignore.

check-wildcard

This option is used to check for non-terminal wildcards. The use of non-terminal wildcards is almost always as a result of a failure to understand the wildcard matching algorithm (RFC 1034). This option affects primary zones. The default (yes) is to check for non-terminal wildcards and issue a warning.

check-integrity

This performs post-load zone integrity checks on primary zones. It checks that MX and SRV records refer to address (A or AAAA) records and that glue address records exist for delegated zones. For MX and SRV records, only in-zone hostnames are checked (for out-of-zone hostnames, use named-checkzone). For NS records, only names below top-of-zone are checked (for out-of-zone names and glue consistency checks, use named-checkzone). The default is yes.

The use of the SPF record to publish Sender Policy Framework is deprecated, as the migration from using TXT records to SPF records was abandoned. Enabling this option also checks that a TXT Sender Policy Framework record exists (starts with "v=spf1") if there is an SPF record. Warnings are emitted if the TXT record does not exist; they can be suppressed with check-spf.

check-mx-cname

If check-integrity is set, then fail, warn, or ignore MX records that refer to CNAMES. The default is to warn.

check-srv-cname

If check-integrity is set, then fail, warn, or ignore SRV records that refer to CNAMES. The default is to warn.

check-sibling

When performing integrity checks, also check that sibling glue exists. The default is yes.

check-spf

If check-integrity is set, check that there is a TXT Sender Policy Framework record present (starts with "v=spf1") if there is an SPF record present. The default is warn.

zero-no-soa-ttl

If yes, when returning authoritative negative responses to SOA queries, set the TTL of the SOA record returned in the authority section to zero. The default is yes.

zero-no-soa-ttl-cache

If yes, when caching a negative response to an SOA query set the TTL to zero. The default is no.

update-check-ksk

When set to the default value of yes, check the KSK bit in each key to determine how the key should be used when generating RRSIGs for a secure zone.

Ordinarily, zone-signing keys (that is, keys without the KSK bit set) are used to sign the entire zone, while key-signing keys (keys with the KSK bit set) are only used to sign the DNSKEY RRset at the zone apex. However, if this option is set to no, then the KSK bit is ignored; KSKs are treated as if they were ZSKs and are used to sign the entire zone. This is similar to the dnssec-signzone -z command-line option.

When this option is set to yes, there must be at least two active keys for every algorithm represented in the DNSKEY RRset: at least one KSK and one ZSK per algorithm. If there is any algorithm for which this requirement is not met, this option is ignored for that algorithm.

dnssec-dnskey-kskonly

When this option and update-check-ksk are both set to yes, only key-signing keys (that is, keys with the KSK bit set) are used to sign the DNSKEY RRset at the zone apex. Zone-signing keys (keys without the KSK bit set) are used to sign the remainder of the zone, but not the DNSKEY RRset. This is similar to the dnssec-signzone -x command-line option.

The default is no. If update-check-ksk is set to no, this option is ignored.

try-tcp-refresh

If yes, try to refresh the zone using TCP if UDP queries fail. The default is yes.

dnssec-secure-to-insecure

This allows a dynamic zone to transition from secure to insecure (i.e., signed to unsigned) by deleting all of the DNSKEY records. The default is no. If set to yes, and if the DNSKEY RRset at the zone apex is deleted, all RRSIG and NSEC records are removed from the zone as well.

If the zone uses NSEC3, it is also necessary to delete the NSEC3PARAM RRset from the zone apex; this causes the removal of all corresponding NSEC3 records. (It is expected that this requirement will be eliminated in a future release.)

Note that if a zone has been configured with auto-dnssec maintain and the private keys remain accessible in the key repository, then the zone will be automatically signed again the next time named is started.

Forwarding

The forwarding facility can be used to create a large site-wide cache on a few servers, reducing traffic over links to external name servers. It can also be used to allow queries by servers that do not have direct access to the Internet, but wish to look up exterior names anyway. Forwarding occurs only on those queries for which the server is not authoritative and does not have the answer in its cache.

forward: This option is only meaningful if the forwarders list is not empty. A value of first is the default and causes the server to query the forwarders first; if that does not answer the question, the server then looks for the answer itself. If only is specified, the server only queries the forwarders.
forwarders: This specifies a list of IP addresses to which queries are forwarded. The default is the empty list (no forwarding). Each address in the list can be associated with an optional port number and/or DSCP value, and a default port number and DSCP value can be set for the entire list.

Forwarding can also be configured on a per-domain basis, allowing for the global forwarding options to be overridden in a variety of ways. Particular domains can be set to use different forwarders, or have a different forward only/first behavior, or not forward at all; see the section called “zone Statement Grammar”.

Dual-stack Servers

Dual-stack servers are used as servers of last resort, to work around problems in reachability due the lack of support for either IPv4 or IPv6 on the host machine.

dual-stack-servers: This specifies host names or addresses of machines with access to both IPv4 and IPv6 transports. If a hostname is used, the server must be able to resolve the name using only the transport it has. If the machine is dual-stacked, the dual-stack-servers parameter has no effect unless access to a transport has been disabled on the command line (e.g., named -4).

Access Control

Access to the server can be restricted based on the IP address of the requesting system. See the section called “Address Match Lists” for details on how to specify IP address lists.

allow-notify

This ACL specifies which hosts are allowed to notify this secondary server of zone changes in addition to the zone primaries. allow-notify may also be specified in the zone statement, in which case it overrides the options allow-notify statement. It is only meaningful for a secondary zone. If not specified, the default is to process notify messages only from a zone's primary.

allow-query

This specifies which hosts are allowed to ask ordinary DNS questions. allow-query may also be specified in the zone statement, in which case it overrides the options allow-query statement. If not specified, the default is to allow queries from all hosts.

Note

allow-query-cache is used to specify access to the cache.

allow-query-on

This specifies which local addresses can accept ordinary DNS questions. This makes it possible, for instance, to allow queries on internal-facing interfaces but disallow them on external-facing ones, without necessarily knowing the internal network's addresses.

Note that allow-query-on is only checked for queries that are permitted by allow-query. A query must be allowed by both ACLs, or it is refused.

allow-query-on may also be specified in the zone statement, in which case it overrides the options allow-query-on statement.

If not specified, the default is to allow queries on all addresses.

Note

allow-query-cache is used to specify access to the cache.

allow-query-cache

This specifies which hosts are allowed to get answers from the cache. If allow-query-cache is not set, BIND checks to see if the following parameters are set, in order: allow-recursion and allow-query (unless recursion no; is set, in which case none; is used). If neither of those parameters is set, the default (localnets; localhost;) is used.

allow-query-cache-on

This specifies which local addresses can send answers from the cache. If not specified, the default is to allow cache queries on any address, localnets, and localhost.

allow-recursion

This specifies which hosts are allowed to make recursive queries through this server. BIND checks to see if the following parameters are set, in order: allow-recursion, allow-query-cache, and allow-query. If none of those parameters are set, the default (localnets; localhost;) is used.

allow-recursion-on

This specifies which local addresses can accept recursive queries. If not specified, the default is to allow recursive queries on all addresses.

allow-update

This specifies which hosts are allowed to submit Dynamic DNS updates for primary zones. The default is to deny updates from all hosts. Note that allowing updates based on the requestor's IP address is insecure; see the section called “Dynamic Update Security” for details.

allow-update-forwarding

This specifies which hosts are allowed to submit Dynamic DNS updates to secondary zones to be forwarded to the primary. The default is { none; }, which means that no update forwarding is performed. To enable update forwarding, specify allow-update-forwarding { any; };. Specifying values other than { none; } or { any; } is usually counterproductive; the responsibility for update access control should rest with the primary server, not the secondaries.

Note that enabling the update forwarding feature on a secondary server may expose primary servers to attacks if they rely on insecure IP-address-based access control; see the section called “Dynamic Update Security” for more details.

allow-v6-synthesis

This option was introduced for the smooth transition from AAAA to A6 and from "nibble labels" to binary labels. However, since both A6 and binary labels were then deprecated, this option was also deprecated. It is now ignored with some warning messages.

allow-transfer

This specifies which hosts are allowed to receive zone transfers from the server. allow-transfer may also be specified in the zone statement, in which case it overrides the options allow-transfer statement. If not specified, the default is to allow transfers to all hosts.

blackhole

This specifies a list of addresses which the server does accept queries from or use to resolve a query. Queries from these addresses are not responded to. The default is none.

filter-aaaa

This specifies a list of addresses to which filter-aaaa-on-v4 and filter-aaaa-on-v6 apply. The default is any.

keep-response-order

This specifies a list of addresses to which the server sends responses to TCP queries, in the same order in which they were received. This disables the processing of TCP queries in parallel. The default is none.

no-case-compress

This specifies a list of addresses which require responses to use case-insensitive compression. This ACL can be used when named needs to work with clients that do not comply with the requirement in RFC 1034 to use case-insensitive name comparisons when checking for matching domain names.

If left undefined, the ACL defaults to none: case-insensitive compression is used for all clients. If the ACL is defined and matches a client, case is ignored when compressing domain names in DNS responses sent to that client.

This can result in slightly smaller responses; if a response contains the names "example.com" and "example.COM", case-insensitive compression treats the second one as a duplicate. It also ensures that the case of the query name exactly matches the case of the owner names of returned records, rather than matches the case of the records entered in the zone file. This allows responses to exactly match the query, which is required by some clients due to incorrect use of case-sensitive comparisons.

Case-insensitive compression is always used in AXFR and IXFR responses, regardless of whether the client matches this ACL.

There are circumstances in which named does not preserve the case of owner names of records: if a zone file defines records of different types with the same name, but the capitalization of the name is different (e.g., "www.example.com/A" and "WWW.EXAMPLE.COM/AAAA"), then all responses for that name use the first version of the name that was used in the zone file. This limitation may be addressed in a future release. However, domain names specified in the rdata of resource records (i.e., records of type NS, MX, CNAME, etc.) always have their case preserved unless the client matches this ACL.

resolver-query-timeout

This is the amount of time in seconds that the resolver spends attempting to resolve a recursive query before failing. The default and minimum is 10 and the maximum is 30. Setting it to 0 results in the default being used.

Interfaces

The interfaces and ports that the server answers queries from may be specified using the listen-on option. listen-on takes an optional port and an address_match_list of IPv4 addresses. (IPv6 addresses are ignored, with a logged warning.) The server listens on all interfaces allowed by the address match list. If a port is not specified, port 53 is used.

Multiple listen-on statements are allowed. For example:

listen-on { 5.6.7.8; };
listen-on port 1234 { !1.2.3.4; 1.2/16; };

enables the name server on port 53 for the IP address 5.6.7.8, and on port 1234 of an address on the machine in net 1.2 that is not 1.2.3.4.

If no listen-on is specified, the server listens on port 53 on all IPv4 interfaces.

The listen-on-v6 option is used to specify the interfaces and the ports on which the server listens for incoming queries sent using IPv6. If not specified, the server listens on port 53 on all IPv6 interfaces.

When

{ any; }

is specified as the address_match_list for the listen-on-v6 option, the server does not bind a separate socket to each IPv6 interface address as it does for IPv4, if the operating system has enough API support for IPv6 (specifically, if it conforms to RFC 3493 and RFC 3542). Instead, it listens on the IPv6 wildcard address. If the system only has incomplete API support for IPv6, however, the behavior is the same as that for IPv4.

A list of particular IPv6 addresses can also be specified, in which case the server listens on a separate socket for each specified address, regardless of whether the desired API is supported by the system. IPv4 addresses specified in listen-on-v6 are ignored, with a logged warning.

Multiple listen-on-v6 options can be used. For example:

listen-on-v6 { any; };
listen-on-v6 port 1234 { !2001:db8::/32; any; };

enables the name server on port 53 for any IPv6 addresses (with a single wildcard socket), and on port 1234 of IPv6 addresses that are not in the prefix 2001:db8::/32 (with separate sockets for each matched address).

To instruct the server not to listen on any IPv6 address, use:

listen-on-v6 { none; };

Query Address

If the server does not know the answer to a question, it queries other name servers. query-source specifies the address and port used for such queries. For queries sent over IPv6, there is a separate query-source-v6 option. If address is * (asterisk) or is omitted, a wildcard IP address (INADDR_ANY) is used.

If port is * or is omitted, a random port number from a pre-configured range is picked up and used for each query. The port range(s) is specified in the use-v4-udp-ports (for IPv4) and use-v6-udp-ports (for IPv6) options, excluding the ranges specified in the avoid-v4-udp-ports and avoid-v6-udp-ports options, respectively.

The defaults of the query-source and query-source-v6 options are:

query-source address * port *;
query-source-v6 address * port *;

If use-v4-udp-ports or use-v6-udp-ports is unspecified, named checks whether the operating system provides a programming interface to retrieve the system's default range for ephemeral ports. If such an interface is available, named uses the corresponding system default range; otherwise, it uses its own defaults:

use-v4-udp-ports { range 1024 65535; };
use-v6-udp-ports { range 1024 65535; };

Note: make sure the ranges are sufficiently large for security. A desirable size depends on several parameters, but we generally recommend it contain at least 16384 ports (14 bits of entropy). Note also that the system's default range when used may be too small for this purpose, and that the range may even be changed while named is running; the new range is automatically applied when named is reloaded. Explicit configuration of use-v4-udp-ports and use-v6-udp-ports is encouraged, so that the ranges are sufficiently large and are reasonably independent from the ranges used by other applications.

Note: the operational configuration where named runs may prohibit the use of some ports. For example, Unix systems do not allow named, if run without root privilege, to use ports less than 1024. If such ports are included in the specified (or detected) set of query ports, the corresponding query attempts will fail, resulting in resolution failures or delay. It is therefore important to configure the set of ports that can be safely used in the expected operational environment.

The defaults of the avoid-v4-udp-ports and avoid-v6-udp-ports options are:

avoid-v4-udp-ports {};
avoid-v6-udp-ports {};

Note: BIND 9.5.0 introduced the use-queryport-pool option to support a pool of such random ports, but this option is now obsolete because reusing the same ports in the pool may not be sufficiently secure. For the same reason, it is generally strongly discouraged to specify a particular port for the query-source or query-source-v6 options; it implicitly disables the use of randomized port numbers.

use-queryport-pool: This option is obsolete.
queryport-pool-ports: This option is obsolete.
queryport-pool-updateinterval: This option is obsolete.

Note

The address specified in the query-source option is used for both UDP and TCP queries, but the port applies only to UDP queries. TCP queries always use a random unprivileged port.

Note

Solaris 2.5.1 and earlier does not support setting the source address for TCP sockets.

Note

See also transfer-source and notify-source.

Zone Transfers

BIND has mechanisms in place to facilitate zone transfers and set limits on the amount of load that transfers place on the system. The following options apply to zone transfers.

also-notify

This option defines a global list of IP addresses of name servers that are also sent NOTIFY messages whenever a fresh copy of the zone is loaded, in addition to the servers listed in the zone's NS records. This helps to ensure that copies of the zones quickly converge on stealth servers. Optionally, a port may be specified with each also-notify address to send the notify messages to a port other than the default of 53. An optional TSIG key can also be specified with each address to cause the notify messages to be signed; this can be useful when sending notifies to multiple views. In place of explicit addresses, one or more named masters lists can be used.

If an also-notify list is given in a zone statement, it overrides the options also-notify statement. When a zone notify statement is set to no, the IP addresses in the global also-notify list are not sent NOTIFY messages for that zone. The default is the empty list (no global notification list).

max-transfer-time-in

Inbound zone transfers running longer than this many minutes are terminated. The default is 120 minutes (2 hours). The maximum value is 28 days (40320 minutes).

max-transfer-idle-in

Inbound zone transfers making no progress in this many minutes are terminated. The default is 60 minutes (1 hour). The maximum value is 28 days (40320 minutes).

max-transfer-time-out

Outbound zone transfers running longer than this many minutes are terminated. The default is 120 minutes (2 hours). The maximum value is 28 days (40320 minutes).

max-transfer-idle-out

Outbound zone transfers making no progress in this many minutes are terminated. The default is 60 minutes (1 hour). The maximum value is 28 days (40320 minutes).

notify-rate

This specifies the rate at which NOTIFY requests are sent during normal zone maintenance operations. (NOTIFY requests due to initial zone loading are subject to a separate rate limit; see below.) The default is 20 per second. The lowest possible rate is one per second; when set to zero, it is silently raised to one.

startup-notify-rate

This is the rate at which NOTIFY requests are sent when the name server is first starting up, or when zones have been newly added to the name server. The default is 20 per second. The lowest possible rate is one per second; when set to zero, it is silently raised to one.

serial-query-rate

Secondary servers periodically query primary servers to find out if zone serial numbers have changed. Each such query uses a minute amount of the secondary server's network bandwidth. To limit the amount of bandwidth used, BIND 9 limits the rate at which queries are sent. The value of the serial-query-rate option, an integer, is the maximum number of queries sent per second. The default is 20 per second. The lowest possible rate is one per second; when set to zero, it is silently raised to one.

serial-queries

BIND 9 does not limit the number of outstanding serial queries and ignores the serial-queries option. Instead, it limits the rate at which the queries are sent as defined using the serial-query-rate option.

transfer-format

Zone transfers can be sent using two different formats, one-answer and many-answers. The transfer-format option is used on the primary server to determine which format it sends. one-answer uses one DNS message per resource record transferred. many-answers packs as many resource records as possible into one message. many-answers is more efficient; the default is many-answers. The many-answers format is also supported by recent Microsoft Windows name servers. transfer-format may be overridden on a per-server basis by using the server statement.

transfer-message-size

This is an upper bound on the uncompressed size of DNS messages used in zone transfers over TCP. If a message grows larger than this size, additional messages are used to complete the zone transfer. (Note, however, that this is a hint, not a hard limit; if a message contains a single resource record whose RDATA does not fit within the size limit, a larger message will be permitted so the record can be transferred.)

Valid values are between 512 and 65535 octets; any values outside that range are adjusted to the nearest value within it. The default is 20480, which was selected to improve message compression; most DNS messages of this size will compress to less than 16536 bytes. Larger messages cannot be compressed as effectively, because 16536 is the largest permissible compression offset pointer in a DNS message.

This option is mainly intended for server testing; there is rarely any benefit in setting a value other than the default.

transfers-in

This is the maximum number of inbound zone transfers that can run concurrently. The default value is 10. Increasing transfers-in may speed up the convergence of secondary zones, but it also may increase the load on the local system.

transfers-out

This is the maximum number of outbound zone transfers that can run concurrently. Zone transfer requests in excess of the limit are refused. The default value is 10.

transfers-per-ns

This is the maximum number of inbound zone transfers that can concurrently transfer from a given remote name server. The default value is 2. Increasing transfers-per-ns may speed up the convergence of secondary zones, but it also may increase the load on the remote name server. transfers-per-ns may be overridden on a per-server basis by using the transfers phrase of the server statement.

transfer-source

transfer-source determines which local address is bound to IPv4 TCP connections used to fetch zones transferred inbound by the server. It also determines the source IPv4 address, and optionally the UDP port, used for the refresh queries and forwarded dynamic updates. If not set, it defaults to a system-controlled value which is usually the address of the interface "closest to" the remote end. This address must appear in the remote end's allow-transfer option for the zone being transferred, if one is specified. This statement sets the transfer-source for all zones, but can be overridden on a per-view or per-zone basis by including a transfer-source statement within the view or zone block in the configuration file.

Note

Solaris 2.5.1 and earlier does not support setting the source address for TCP sockets.

transfer-source-v6

This option is the same as transfer-source, except zone transfers are performed using IPv6.

alt-transfer-source

This indicates an alternate transfer source if the one listed in transfer-source fails and use-alt-transfer-source is set.

Note

To avoid using the alternate transfer source, set use-alt-transfer-source appropriately and do not depend upon getting an answer back to the first refresh query.

alt-transfer-source-v6

This indicates an alternate transfer source if the one listed in transfer-source-v6 fails and use-alt-transfer-source is set.

use-alt-transfer-source

This indicates whether the alternate transfer sources should be used. If views are specified, this defaults to no; otherwise, it defaults to yes.

notify-source

notify-source determines which local source address, and optionally UDP port, is used to send NOTIFY messages. This address must appear in the secondary server's masters zone clause or in an allow-notify clause. This statement sets the notify-source for all zones, but can be overridden on a per-zone or per-view basis by including a notify-source statement within the zone or view block in the configuration file.

Note

Solaris 2.5.1 and earlier does not support setting the source address for TCP sockets.

notify-source-v6

This option acts like notify-source, but applies to notify messages sent to IPv6 addresses.

UDP Port Lists

use-v4-udp-ports, avoid-v4-udp-ports, use-v6-udp-ports, and avoid-v6-udp-ports specify a list of IPv4 and IPv6 UDP ports that are or are not used as source ports for UDP messages. See the section called “Query Address” about how the available ports are determined. For example, with the following configuration:

use-v6-udp-ports { range 32768 65535; };
avoid-v6-udp-ports { 40000; range 50000 60000; };

UDP ports of IPv6 messages sent from named are in one of the following ranges: 32768 to 39999, 40001 to 49999, and 60001 to 65535.

avoid-v4-udp-ports and avoid-v6-udp-ports can be used to prevent named from choosing as its random source port a port that is blocked by a firewall or a port that is used by other applications; if a query went out with a source port blocked by a firewall, the answer would not pass through the firewall and the name server would have to query again. Note: the desired range can also be represented only with use-v4-udp-ports and use-v6-udp-ports, and the avoid- options are redundant in that sense; they are provided for backward compatibility and to possibly simplify the port specification.

Operating System Resource Limits

The server's usage of many system resources can be limited. Scaled values are allowed when specifying resource limits. For example, 1G can be used instead of 1073741824 to specify a limit of one gigabyte. unlimited requests unlimited use, or the maximum available amount. default uses the limit that was in force when the server was started. See the description of size_spec in the section called “Configuration File Elements”.

The following options set operating system resource limits for the name server process. Some operating systems do not support some or any of the limits; on such systems, a warning is issued if an unsupported limit is used.

coresize: This sets the maximum size of a core dump. The default is default.
datasize: This sets the maximum amount of data memory the server may use. The default is default. This is a hard limit on server memory usage; if the server attempts to allocate memory in excess of this limit, the allocation will fail, which may in turn leave the server unable to perform DNS service. Therefore, this option is rarely useful as a way to limit the amount of memory used by the server, but it can be used to raise an operating system data size limit that is too small by default. To limit the amount of memory used by the server, use the max-cache-size and recursive-clients options instead.
files: This sets the maximum number of files the server may have open concurrently. The default is unlimited.
stacksize: This sets the maximum amount of stack memory the server may use. The default is default.

Server Resource Limits

The following options set limits on the server's resource consumption that are enforced internally by the server rather than by the operating system.

max-ixfr-log-size

This option is obsolete; it is accepted and ignored for BIND 8 compatibility. The option max-journal-size performs a similar function in BIND 9.

max-journal-size

This sets a maximum size for each journal file (see the section called “The Journal File”). When the journal file approaches the specified size, some of the oldest transactions in the journal are automatically removed. The largest permitted value is 2 gigabytes. The default is unlimited, which also means 2 gigabytes. This option may also be set on a per-zone basis.

max-records

This sets the maximum number of records permitted in a zone. The default is zero, which means the maximum is unlimited.

host-statistics-max

In BIND 8, this specified the maximum number of host statistics entries to be kept. It is not implemented in BIND 9.

recursive-clients

This sets the maximum number (a "hard quota") of simultaneous recursive lookups the server performs on behalf of clients. The default is 1000. Because each recursing client uses a fair bit of memory (on the order of 20 kilobytes), the value of the recursive-clients option may have to be decreased on hosts with limited memory.

recursive-clients defines a "hard quota" limit for pending recursive clients; when more clients than this are pending, new incoming requests are not accepted, and for each incoming request a previous pending request is dropped.

A "soft quota" is also set. When this lower quota is exceeded, incoming requests are accepted, but for each one, a pending request is dropped. If recursive-clients is greater than 1000, the soft quota is set to recursive-clients minus 100; otherwise it is set to 90% of recursive-clients.

tcp-clients

This is the maximum number of simultaneous client TCP connections that the server accepts. The default is 150.

clients-per-query, max-clients-per-query

These set the initial value (minimum) and maximum number of recursive simultaneous clients for any given query (<qname,qtype,qclass>) that the server accepts before dropping additional clients. named attempts to self-tune this value and changes are logged. The default values are 10 and 100.

This value should reflect how many queries come in for a given name in the time it takes to resolve that name. If the number of queries exceeds this value, named assumes that it is dealing with a non-responsive zone and drops additional queries. If it gets a response after dropping queries, it raises the estimate. The estimate is then lowered in 20 minutes if it has remained unchanged.

If clients-per-query is set to zero, there is no limit on the number of clients per query and no queries are dropped.

If max-clients-per-query is set to zero, there is no upper bound other than imposed by recursive-clients.

fetches-per-zone

This sets the maximum number of simultaneous iterative queries to any one domain that the server permits before blocking new queries for data in or beneath that zone. This value should reflect how many fetches would normally be sent to any one zone in the time it would take to resolve them. It should be smaller than recursive-clients.

When many clients simultaneously query for the same name and type, the clients are all attached to the same fetch, up to the max-clients-per-query limit, and only one iterative query is sent. However, when clients are simultaneously querying for different names or types, multiple queries are sent and max-clients-per-query is not effective as a limit.

Optionally, this value may be followed by the keyword drop or fail, indicating whether queries which exceed the fetch quota for a zone are dropped with no response, or answered with SERVFAIL. The default is drop.

If fetches-per-zone is set to zero, there is no limit on the number of fetches per query and no queries are dropped. The default is zero.

The current list of active fetches can be dumped by running rndc recursing. The list includes the number of active fetches for each domain and the number of queries that have been passed or dropped as a result of the fetches-per-zone limit. (Note: these counters are not cumulative over time; whenever the number of active fetches for a domain drops to zero, the counter for that domain is deleted, and the next time a fetch is sent to that domain, it is recreated with the counters set to zero.)

fetches-per-server

This sets the maximum number of simultaneous iterative queries that the server allows to be sent to a single upstream name server before blocking additional queries. This value should reflect how many fetches would normally be sent to any one server in the time it would take to resolve them. It should be smaller than recursive-clients.

Optionally, this value may be followed by the keyword drop or fail, indicating whether queries are dropped with no response or answered with SERVFAIL, when all of the servers authoritative for a zone are found to have exceeded the per-server quota. The default is fail.

If fetches-per-server is set to zero, there is no limit on the number of fetches per query and no queries are dropped. The default is zero.

The fetches-per-server quota is dynamically adjusted in response to detected congestion. As queries are sent to a server and are either answered or time out, an exponentially weighted moving average is calculated of the ratio of timeouts to responses. If the current average timeout ratio rises above a "high" threshold, then fetches-per-server is reduced for that server. If the timeout ratio drops below a "low" threshold, then fetches-per-server is increased. The fetch-quota-params options can be used to adjust the parameters for this calculation.

fetch-quota-params

This sets the parameters to use for dynamic resizing of the fetches-per-server quota in response to detected congestion.

The first argument is an integer value indicating how frequently to recalculate the moving average of the ratio of timeouts to responses for each server. The default is 100, meaning that BIND recalculates the average ratio after every 100 queries have either been answered or timed out.

The remaining three arguments represent the "low" threshold (defaulting to a timeout ratio of 0.1), the "high" threshold (defaulting to a timeout ratio of 0.3), and the discount rate for the moving average (defaulting to 0.7). A higher discount rate causes recent events to weigh more heavily when calculating the moving average; a lower discount rate causes past events to weigh more heavily, smoothing out short-term blips in the timeout ratio. These arguments are all fixed-point numbers with precision of 1/100; at most two places after the decimal point are significant.

reserved-sockets

This sets the number of file descriptors reserved for TCP, stdio, etc. This needs to be big enough to cover the number of interfaces named listens on plus tcp-clients, as well as to provide room for outgoing TCP queries and incoming zone transfers. The default is 512. The minimum value is 128 and the maximum value is 128 fewer than maxsockets (-S). This option may be removed in the future.

This option has little effect on Windows.

max-cache-size

This sets the maximum amount of memory to use for the server's cache, in bytes or percentage of total physical memory. When the amount of data in the cache reaches this limit, the server causes records to expire prematurely, following an LRU-based strategy, so that the limit is not exceeded. The keyword unlimited, or the value 0, places no limit on the cache size; records are purged from the cache only when their TTLs expire. Any positive values less than 2MB are ignored and reset to 2MB. In a server with multiple views, the limit applies separately to the cache of each view. The default is 90%. On systems where detection of the amount of physical memory is not supported, values represented as a percentage fall back to unlimited. Note that the detection of physical memory is done only once at startup, so named does not adjust the cache size if the amount of physical memory is changed during runtime.

tcp-listen-queue

This sets the listen-queue depth. The default and minimum is 10. If the kernel supports the accept filter "dataready", this also controls how many TCP connections are queued in kernel space waiting for some data before being passed to accept. Non-zero values less than 10 are silently raised. A value of 0 may also be used; on most platforms this sets the listen-queue length to a system-defined default value.

Periodic Task Intervals

cleaning-interval: This interval is effectively obsolete. Previously, the server removed expired resource records from the cache every cleaning-interval minutes. BIND 9 now manages cache memory in a more sophisticated manner and does not rely on periodic cleaning anymore. Specifying this option therefore has no effect on the server's behavior.
heartbeat-interval: The server performs zone maintenance tasks for all zones marked as dialup whenever this interval expires. The default is 60 minutes. Reasonable values are up to 1 day (1440 minutes). The maximum value is 28 days (40320 minutes). If set to 0, no zone maintenance for these zones occurs.
interface-interval: The server scans the network interface list every interface-interval minutes. The default is 60 minutes; the maximum value is 28 days (40320 minutes). If set to 0, interface scanning only occurs when the configuration file is loaded, or when automatic-interface-scan is enabled and supported by the operating system. After the scan, the server begins listening for queries on any newly discovered interfaces (provided they are allowed by the listen-on configuration), and stops listening on interfaces that have gone away.
statistics-interval: Name server statistics are logged every statistics-interval minutes. The default is 60, and the maximum value is 28 days (40320 minutes). If set to 0, no statistics are logged.

Note

This option is not implemented in BIND 9.
topology: In BIND 8, this option indicated network topology so that preferential treatment could be given to the topologically closest name servers when sending queries. It is not implemented in BIND 9.

The sortlist Statement

The response to a DNS query may consist of multiple resource records (RRs) forming a resource record set (RRset). The name server normally returns the RRs within the RRset in an indeterminate order (but see the rrset-order statement in the section called “RRset Ordering”). The client resolver code should rearrange the RRs as appropriate: that is, using any addresses on the local net in preference to other addresses. However, not all resolvers can do this or are correctly configured. When a client is using a local server, the sorting can be performed in the server, based on the client's address. This only requires configuring the name servers, not all the clients.

The sortlist statement (see below) takes an address_match_list and interprets it in a special way. Each top-level statement in the sortlist must itself be an explicit address_match_list with one or two elements. The first element (which may be an IP address, an IP prefix, an ACL name, or a nested address_match_list) of each top-level list is checked against the source address of the query until a match is found. When the addresses in the first element overlap, the first rule to match is selected.

Once the source address of the query has been matched, if the top-level statement contains only one element, the actual primitive element that matched the source address is used to select the address in the response to move to the beginning of the response. If the statement is a list of two elements, then the second element is interpreted as a topology preference list. Each top-level element is assigned a distance, and the address in the response with the minimum distance is moved to the beginning of the response.

In the following example, any queries received from any of the addresses of the host itself get responses preferring addresses on any of the locally connected networks. Next most preferred are addresses on the 192.168.1/24 network, and after that either the 192.168.2/24 or 192.168.3/24 network, with no preference shown between these two networks. Queries received from a host on the 192.168.1/24 network prefer other addresses on that network to the 192.168.2/24 and 192.168.3/24 networks. Queries received from a host on the 192.168.4/24 or the 192.168.5/24 network only prefer other addresses on their directly connected networks.

sortlist {
    // IF the local host
    // THEN first fit on the following nets
    { localhost;
        { localnets;
            192.168.1/24;
            { 192.168.2/24; 192.168.3/24; }; }; };
    // IF on class C 192.168.1 THEN use .1, or .2 or .3
    { 192.168.1/24;
        { 192.168.1/24;
            { 192.168.2/24; 192.168.3/24; }; }; };
    // IF on class C 192.168.2 THEN use .2, or .1 or .3
    { 192.168.2/24;
        { 192.168.2/24;
            { 192.168.1/24; 192.168.3/24; }; }; };
    // IF on class C 192.168.3 THEN use .3, or .1 or .2
    { 192.168.3/24;
        { 192.168.3/24;
            { 192.168.1/24; 192.168.2/24; }; }; };
    // IF .4 or .5 THEN prefer that net
    { { 192.168.4/24; 192.168.5/24; };
    };
};

The following example illustrates reasonable behavior for the local host and hosts on directly connected networks. Responses sent to queries from the local host favor any of the directly connected networks. Responses sent to queries from any other hosts on a directly connected network prefer addresses on that same network. Responses to other queries are not sorted.

sortlist {
           { localhost; localnets; };
           { localnets; };
};

RRset Ordering

Note

While alternating the order of records in a DNS response between subsequent queries is a known load distribution technique, certain caveats apply (mostly stemming from caching) which usually make it a suboptimal choice for load balancing purposes when used on its own.

The rrset-order statement permits configuration of the ordering of the records in a multiple-record response. See also: the section called “The sortlist Statement”.

Each rule in an rrset-order statement is defined as follows:

[class <class_name>] [type <type_name>] [name "<domain_name>"] order <ordering>

The default qualifiers for each rule are:

If no class is specified, the default is ANY.
If no type is specified, the default is ANY.
If no name is specified, the default is * (asterisk).

<domain_name> only matches the name itself, not any of its subdomains. To make a rule match all subdomains of a given name, a wildcard name (*.<domain_name>) must be used. Note that *.<domain_name> does not match <domain_name> itself; to specify RRset ordering for a name and all of its subdomains, two separate rules must be defined: one for <domain_name> and one for *.<domain_name>.

The legal values for <ordering> are:

fixed: Records are returned in the order they are defined in the zone file.

Note

The fixed option is only available if BIND is configured with --enable-fixed-rrset at compile time.
random: Records are returned in a random order.
cyclic: Records are returned in a cyclic round-robin order, rotating by one record per query.

By default, records are returned in random order.

Note that if multiple rrset-order statements are present in the configuration file (at both the options and view levels), they are not combined; instead, the more-specific one (view) replaces the less-specific one (options).

If multiple rules within a single rrset-order statement match a given RRset, the first matching rule is applied.

Example:

rrset-order {
    type A name "foo.isc.org" order random;
    type AAAA name "foo.isc.org" order cyclic;
    name "bar.isc.org" order fixed;
    name "*.bar.isc.org" order random;
    name "*.baz.isc.org" order cyclic;
};

With the above configuration, the following RRset ordering is used:

QNAME	QTYPE	RRset Order
`foo.isc.org`	`A`	random
`foo.isc.org`	`AAAA`	cyclic
`foo.isc.org`	`TXT`	random
`sub.foo.isc.org`	all	random
`bar.isc.org`	all	fixed
`sub.bar.isc.org`	all	random
`baz.isc.org`	all	random
`sub.baz.isc.org`	all	cyclic

Tuning

lame-ttl

This is always set to 0. More information is available in the security advisory for CVE-2021-25219.

servfail-ttl

This sets the number of seconds to cache a SERVFAIL response due to DNSSEC validation failure or other general server failure. If set to 0, SERVFAIL caching is disabled. The SERVFAIL cache is not consulted if a query has the CD (Checking Disabled) bit set; this allows a query that failed due to DNSSEC validation to be retried without waiting for the SERVFAIL TTL to expire.

The maximum value is 30 seconds; any higher value is silently reduced. The default is 1 second.

max-ncache-ttl

To reduce network traffic and increase performance, the server stores negative answers. max-ncache-ttl is used to set a maximum retention time for these answers in the server, in seconds. The default max-ncache-ttl is 10800 seconds (3 hours). max-ncache-ttl cannot exceed 7 days and is silently truncated to 7 days if set to a greater value.

max-cache-ttl

This sets the maximum time for which the server caches ordinary (positive) answers, in seconds. The default is 604800 (one week). A value of zero may cause all queries to return SERVFAIL, because of lost caches of intermediate RRsets (such as NS and glue AAAA/A records) in the resolution process.

min-roots

This sets the minimum number of root servers that is required for a request for the root servers to be accepted. The default is 2.

Note

This is not implemented in BIND 9.

sig-validity-interval

This specifies the number of days into the future that DNSSEC signatures that are automatically generated as a result of dynamic updates (the section called “Dynamic Update”) will expire. There is an optional second field which specifies how long before expiry that the signatures are regenerated. If not specified, the signatures are regenerated at 1/4 of base interval. The second field is specified in days if the base interval is greater than 7 days; otherwise it is specified in hours. The default base interval is 30 days, giving a re-signing interval of 7 1/2 days. The maximum value is 10 years (3660 days).

The signature inception time is unconditionally set to one hour before the current time, to allow for a limited amount of clock skew.

The sig-validity-interval should be at least several multiples of the SOA expire interval, to allow for reasonable interaction between the various timer and expiry dates.

sig-signing-nodes

This specifies the maximum number of nodes to be examined in each quantum, when signing a zone with a new DNSKEY. The default is 100.

sig-signing-signatures

This specifies a threshold number of signatures that terminates processing a quantum, when signing a zone with a new DNSKEY. The default is 10.

sig-signing-type

This specifies a private RDATA type to be used when generating signing-state records. The default is 65534.

This parameter may be removed in a future version, once there is a standard type.

Signing-state records are used internally by named to track the current state of a zone-signing process, i.e., whether it is still active or has been completed. The records can be inspected using the command rndc signing -list zone. Once named has finished signing a zone with a particular key, the signing-state record associated with that key can be removed from the zone by running rndc signing -clear keyid/algorithm zone. To clear all of the completed signing-state records for a zone, use rndc signing -clear all zone.

min-refresh-time, max-refresh-time, min-retry-time, max-retry-time

These options control the server's behavior on refreshing a zone (querying for SOA changes) or retrying failed transfers. Usually the SOA values for the zone are used, up to a hard-coded maximum expiry of 24 weeks. However, these values are set by the primary, giving secondary server administrators little control over their contents.

These options allow the administrator to set a minimum and maximum refresh and retry time in seconds per-zone, per-view, or globally. These options are valid for secondary and stub zones, and clamp the SOA refresh and retry times to the specified values.

The following defaults apply: min-refresh-time 300 seconds, max-refresh-time 2419200 seconds (4 weeks), min-retry-time 500 seconds, and max-retry-time 1209600 seconds (2 weeks).

edns-udp-size

This sets the maximum advertised EDNS UDP buffer size, in bytes, to control the size of packets received from authoritative servers in response to recursive queries. Valid values are 512 to 4096; values outside this range are silently adjusted to the nearest value within it. The default value is 1232.

The usual reason for setting edns-udp-size to a non-default value is to get UDP answers to pass through broken firewalls that block fragmented packets and/or block UDP DNS packets that are greater than 512 bytes.

When named first queries a remote server, it advertises a UDP buffer size of 512, as this has the greatest chance of success on the first try.

If the initial response times out, named tries again with plain DNS; if that is successful, it is taken as evidence that the server does not support EDNS. After enough failures using EDNS and successes using plain DNS, named defaults to plain DNS for future communications with that server. If that happens, named periodically sends an EDNS query to see if the situation has improved.

However, if the initial query is successful with EDNS advertising a buffer size of 512, then named advertises progressively larger buffer sizes on successive queries, until responses begin timing out or edns-udp-size is reached.

The default buffer sizes used by named are 512, 1232, 1432, and 4096, but never exceed edns-udp-size. (The values 1232 and 1432 are chosen to allow for an IPv4-/IPv6-encapsulated UDP message to be sent without fragmentation at the minimum MTU sizes for Ethernet and IPv6 networks.)

max-udp-size

This sets the maximum EDNS UDP message size that named sends, in bytes. Valid values are 512 to 4096; values outside this range are silently adjusted to the nearest value within it. The default value is 1232.

This value applies to responses sent by a server; to set the advertised buffer size in queries, see edns-udp-size.

The usual reason for setting max-udp-size to a non-default value is to allow UDP answers to pass through broken firewalls that block fragmented packets and/or block UDP packets that are greater than 512 bytes. This is independent of the advertised receive buffer (edns-udp-size).

Setting this to a low value encourages additional TCP traffic to the name server.

masterfile-format

This specifies the file format of zone files (see the section called “Additional File Formats”). The default value is text, which is the standard textual representation, except for secondary zones, in which the default value is raw. Files in formats other than text are typically expected to be generated by the named-compilezone tool, or dumped by named.

Note that when a zone file in a format other than text is loaded, named may omit some of the checks which would be performed for a file in text format. In particular, check-names checks do not apply for the raw format. This means a zone file in the raw format must be generated with the same check level as that specified in the named configuration file. Also, map format files are loaded directly into memory via memory mapping, with only minimal checking.

This statement sets the masterfile-format for all zones, but can be overridden on a per-zone or per-view basis by including a masterfile-format statement within the zone or view block in the configuration file.

masterfile-style

This specifies the formatting of zone files during dump, when the masterfile-format is text. This option is ignored with any other masterfile-format.

When set to relative, records are printed in a multi-line format, with owner names expressed relative to a shared origin. When set to full, records are printed in a single-line format with absolute owner names. The full format is most suitable when a zone file needs to be processed automatically by a script. The relative format is more human-readable, and is thus suitable when a zone is to be edited by hand. The default is relative.

max-recursion-depth

This sets the maximum number of levels of recursion that are permitted at any one time while servicing a recursive query. Resolving a name may require looking up a name server address, which in turn requires resolving another name, etc.; if the number of recursions exceeds this value, the recursive query is terminated and returns SERVFAIL. The default is 7.

max-recursion-queries

This sets the maximum number of iterative queries that may be sent while servicing a recursive query. If more queries are sent, the recursive query is terminated and returns SERVFAIL. The default is 100.

notify-delay

This sets the delay, in seconds, between sending sets of NOTIFY messages for a zone. The default is 5 seconds.

The overall rate at which NOTIFY messages are sent for all zones is controlled by serial-query-rate.

max-rsa-exponent-size

This sets the maximum RSA exponent size, in bits, that is accepted when validating. Valid values are 35 to 4096 bits. The default, zero, is also accepted and is equivalent to 4096.

prefetch

When a query is received for cached data which is to expire shortly, named can refresh the data from the authoritative server immediately, ensuring that the cache always has an answer available.

prefetch specifies the "trigger" TTL value at which prefetch of the current query takes place; when a cache record with a lower TTL value is encountered during query processing, it is refreshed. Valid trigger TTL values are 1 to 10 seconds. Values larger than 10 seconds are silently reduced to 10. Setting a trigger TTL to zero causes prefetch to be disabled. The default trigger TTL is 2.

An optional second argument specifies the "eligibility" TTL: the smallest original TTL value that is accepted for a record to be eligible for prefetching. The eligibility TTL must be at least six seconds longer than the trigger TTL; if not, named silently adjusts it upward. The default eligibility TTL is 9.

v6-bias

When determining the next name server to try, this indicates by how many milliseconds to prefer IPv6 name servers. The default is 50 milliseconds.

Built-in Server Information Zones

The server provides some helpful diagnostic information through a number of built-in zones under the pseudo-top-level-domain bind in the CHAOS class. These zones are part of a built-in view (see the section called “view Statement Grammar”) of class CHAOS, which is separate from the default view of class IN. Most global configuration options (allow-query, etc.) apply to this view, but some are locally overridden: notify, recursion, and allow-new-zones are always set to no, and rate-limit is set to allow three responses per second.

To disable these zones, use the options below or hide the built-in CHAOS view by defining an explicit view of class CHAOS that matches all clients.

version

This is the version the server should report via a query of the name version.bind with type TXT and class CHAOS. The default is the real version number of this server. Specifying version none disables processing of the queries.

Setting version to any value (including none) also disables queries for authors.bind TXT CH.

hostname

This is the hostname the server should report via a query of the name hostname.bind with type TXT and class CHAOS. This defaults to the hostname of the machine hosting the name server, as found by the gethostname() function. The primary purpose of such queries is to identify which of a group of anycast servers is actually answering the queries. Specifying hostname none; disables processing of the queries.

server-id

This is the ID the server should report when receiving a Name Server Identifier (NSID) query, or a query of the name ID.SERVER with type TXT and class CHAOS. The primary purpose of such queries is to identify which of a group of anycast servers is actually answering the queries. Specifying server-id none; disables processing of the queries. Specifying server-id hostname; causes named to use the hostname as found by the gethostname() function. The default server-id is none.

Built-in Empty Zones

The named server has some built-in empty zones, for SOA and NS records only. These are for zones that should normally be answered locally and which queries should not be sent to the Internet's root servers. The official servers which cover these namespaces return NXDOMAIN responses to these queries. In particular, these cover the reverse namespaces for addresses from RFC 1918, RFC 4193, RFC 5737, and RFC 6598. They also include the reverse namespace for the IPv6 local address (locally assigned), IPv6 link local addresses, the IPv6 loopback address, and the IPv6 unknown address.

The server attempts to determine if a built-in zone already exists or is active (covered by a forward-only forwarding declaration) and does not create an empty zone if either is true.

The current list of empty zones is:

10.IN-ADDR.ARPA
16.172.IN-ADDR.ARPA
17.172.IN-ADDR.ARPA
18.172.IN-ADDR.ARPA
19.172.IN-ADDR.ARPA
20.172.IN-ADDR.ARPA
21.172.IN-ADDR.ARPA
22.172.IN-ADDR.ARPA
23.172.IN-ADDR.ARPA
24.172.IN-ADDR.ARPA
25.172.IN-ADDR.ARPA
26.172.IN-ADDR.ARPA
27.172.IN-ADDR.ARPA
28.172.IN-ADDR.ARPA
29.172.IN-ADDR.ARPA
30.172.IN-ADDR.ARPA
31.172.IN-ADDR.ARPA
168.192.IN-ADDR.ARPA
64.100.IN-ADDR.ARPA
65.100.IN-ADDR.ARPA
66.100.IN-ADDR.ARPA
67.100.IN-ADDR.ARPA
68.100.IN-ADDR.ARPA
69.100.IN-ADDR.ARPA
70.100.IN-ADDR.ARPA
71.100.IN-ADDR.ARPA
72.100.IN-ADDR.ARPA
73.100.IN-ADDR.ARPA
74.100.IN-ADDR.ARPA
75.100.IN-ADDR.ARPA
76.100.IN-ADDR.ARPA
77.100.IN-ADDR.ARPA
78.100.IN-ADDR.ARPA
79.100.IN-ADDR.ARPA
80.100.IN-ADDR.ARPA
81.100.IN-ADDR.ARPA
82.100.IN-ADDR.ARPA
83.100.IN-ADDR.ARPA
84.100.IN-ADDR.ARPA
85.100.IN-ADDR.ARPA
86.100.IN-ADDR.ARPA
87.100.IN-ADDR.ARPA
88.100.IN-ADDR.ARPA
89.100.IN-ADDR.ARPA
90.100.IN-ADDR.ARPA
91.100.IN-ADDR.ARPA
92.100.IN-ADDR.ARPA
93.100.IN-ADDR.ARPA
94.100.IN-ADDR.ARPA
95.100.IN-ADDR.ARPA
96.100.IN-ADDR.ARPA
97.100.IN-ADDR.ARPA
98.100.IN-ADDR.ARPA
99.100.IN-ADDR.ARPA
100.100.IN-ADDR.ARPA
101.100.IN-ADDR.ARPA
102.100.IN-ADDR.ARPA
103.100.IN-ADDR.ARPA
104.100.IN-ADDR.ARPA
105.100.IN-ADDR.ARPA
106.100.IN-ADDR.ARPA
107.100.IN-ADDR.ARPA
108.100.IN-ADDR.ARPA
109.100.IN-ADDR.ARPA
110.100.IN-ADDR.ARPA
111.100.IN-ADDR.ARPA
112.100.IN-ADDR.ARPA
113.100.IN-ADDR.ARPA
114.100.IN-ADDR.ARPA
115.100.IN-ADDR.ARPA
116.100.IN-ADDR.ARPA
117.100.IN-ADDR.ARPA
118.100.IN-ADDR.ARPA
119.100.IN-ADDR.ARPA
120.100.IN-ADDR.ARPA
121.100.IN-ADDR.ARPA
122.100.IN-ADDR.ARPA
123.100.IN-ADDR.ARPA
124.100.IN-ADDR.ARPA
125.100.IN-ADDR.ARPA
126.100.IN-ADDR.ARPA
127.100.IN-ADDR.ARPA
0.IN-ADDR.ARPA
127.IN-ADDR.ARPA
254.169.IN-ADDR.ARPA
2.0.192.IN-ADDR.ARPA
100.51.198.IN-ADDR.ARPA
113.0.203.IN-ADDR.ARPA
255.255.255.255.IN-ADDR.ARPA
0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA
1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA
8.B.D.0.1.0.0.2.IP6.ARPA
D.F.IP6.ARPA
8.E.F.IP6.ARPA
9.E.F.IP6.ARPA
A.E.F.IP6.ARPA
B.E.F.IP6.ARPA
EMPTY.AS112.ARPA
HOME.ARPA

Empty zones can be set at the view level and only apply to views of class IN. Disabled empty zones are only inherited from options if there are no disabled empty zones specified at the view level. To override the options list of disabled zones, disable the root zone at the view level. For example:

            disable-empty-zone ".";

If using the address ranges covered here, reverse zones covering the addresses should already be in place. In practice this appears to not be the case, with many queries being made to the infrastructure servers for names in these spaces. So many, in fact, that sacrificial servers had to be deployed to channel the query load away from the infrastructure servers.

Note

The real parent servers for these zones should disable all empty zones under the parent zone they serve. For the real root servers, this is all built-in empty zones. This enables them to return referrals to deeper in the tree.

empty-server: This specifies the server name that appears in the returned SOA record for empty zones. If none is specified, the zone's name is used.
empty-contact: This specifies the contact name that appears in the returned SOA record for empty zones. If none is specified, "." is used.
empty-zones-enable: This enables or disables all empty zones. By default, they are enabled.
disable-empty-zone: This disables individual empty zones. By default, none are disabled. This option can be specified multiple times.

Additional Section Caching

The additional section cache, also called acache, is an internal cache to improve the response performance of BIND 9. When additional section caching is enabled, BIND 9 caches an internal shortcut to the additional section content for each answer RR. Note that acache is an internal caching mechanism of BIND 9, and is not related to the DNS caching server function.

Additional section caching does not change the response content (except the RRsets ordering of the additional section; see below), but can improve the response performance significantly. It is particularly effective when BIND 9 acts as an authoritative server for a zone that has many delegations with many glue RRs.

To obtain the maximum performance improvement from additional section caching, setting additional-from-cache to no is recommended, since the current implementation of acache does not shortcut additional section information from the DNS cache data.

One obvious disadvantage of acache is that it requires much more memory for the internal cached data. Thus, if the response performance does not matter and memory consumption is more critical, the acache mechanism can be disabled by setting acache-enable to no. It is also possible to specify the upper limit of memory consumption for acache by using max-acache-size.

Additional section caching also has a minor effect on the RRset ordering in the additional section. Without acache, cyclic order is effective for the additional section as well as for the answer and authority sections. However, additional section caching fixes the ordering when it first caches an RRset for the additional section, and the same ordering is kept in succeeding responses, regardless of the setting of rrset-order. The effect of this should be minor, however, since an RRset in the additional section typically only contains a small number of RRs (and in many cases only a single RR), so the ordering is not significant.

The following is a summary of options related to acache.

acache-enable: If yes, additional section caching is enabled. The default value is no.
acache-cleaning-interval: The server removes stale cache entries, based on an LRU-based algorithm, every acache-cleaning-interval minutes. The default is 60 minutes. If set to 0, no periodic cleaning occurs.
max-acache-size: This is the maximum amount of memory, in bytes, to use for the server's acache. When the amount of data in the acache reaches this limit, the server cleans more aggressively so that the limit is not exceeded. In a server with multiple views, the limit applies separately to the acache of each view. The default is 16M.

Content Filtering

BIND 9 provides the ability to filter out responses from external DNS servers containing certain types of data in the answer section. Specifically, it can reject address (A or AAAA) records if the corresponding IPv4 or IPv6 addresses match the given address_match_list of the deny-answer-addresses option. It can also reject CNAME or DNAME records if the "alias" name (i.e., the CNAME alias or the substituted query name due to DNAME) matches the given namelist of the deny-answer-aliases option, where "match" means the alias name is a subdomain of one of the name_list elements. If the optional namelist is specified with except-from, records whose query name matches the list are accepted regardless of the filter setting. Likewise, if the alias name is a subdomain of the corresponding zone, the deny-answer-aliases filter does not apply; for example, even if "example.com" is specified for deny-answer-aliases,

www.example.com. CNAME xxx.example.com.

returned by an "example.com" server is accepted.

In the address_match_list of the deny-answer-addresses option, only ip_addr and ip_prefix are meaningful; any key_id is silently ignored.

If a response message is rejected due to the filtering, the entire message is discarded without being cached, and a SERVFAIL error is returned to the client.

This filtering is intended to prevent "DNS rebinding attacks," in which an attacker, in response to a query for a domain name the attacker controls, returns an IP address within the user's own network or an alias name within the user's own domain. A naive web browser or script could then serve as an unintended proxy, allowing the attacker to get access to an internal node of the local network that could not be externally accessed otherwise. See the paper available at https://dl.acm.org/doi/10.1145/1315245.1315298 for more details about these attacks.

For example, with a domain named "example.net" and an internal network using an IPv4 prefix 192.0.2.0/24, an administrator might specify the following rules:

deny-answer-addresses { 192.0.2.0/24; } except-from { "example.net"; };
deny-answer-aliases { "example.net"; };

If an external attacker let a web browser in the local network look up an IPv4 address of "attacker.example.com", the attacker's DNS server would return a response like this:

attacker.example.com. A 192.0.2.1

in the answer section. Since the rdata of this record (the IPv4 address) matches the specified prefix 192.0.2.0/24, this response would be ignored.

On the other hand, if the browser looked up a legitimate internal web server "www.example.net" and the following response were returned to the BIND 9 server:

www.example.net. A 192.0.2.2

it would be accepted, since the owner name "www.example.net" matches the except-from element, "example.net".

Note that this is not really an attack on the DNS per se. In fact, there is nothing wrong with having an "external" name mapped to an "internal" IP address or domain name from the DNS point of view; it might actually be provided for a legitimate purpose, such as for debugging. As long as the mapping is provided by the correct owner, it either is not possible or does not make sense to detect whether the intent of the mapping is legitimate within the DNS. The "rebinding" attack must primarily be protected at the application that uses the DNS. For a large site, however, it may be difficult to protect all possible applications at once. This filtering feature is provided only to help such an operational environment; turning it on is generally discouraged unless there is no other choice and the attack is a real threat to applications.

Care should be particularly taken if using this option for addresses within 127.0.0.0/8. These addresses are obviously "internal," but many applications conventionally rely on a DNS mapping from some name to such an address. Filtering out DNS records containing this address spuriously can break such applications.

Response Policy Zone (RPZ) Rewriting

BIND 9 includes a limited mechanism to modify DNS responses for requests analogous to email anti-spam DNS rejection lists. Responses can be changed to deny the existence of domains (NXDOMAIN), deny the existence of IP addresses for domains (NODATA), or contain other IP addresses or data.

Response policy zones are named in the response-policy option for the view or among the global options if there is no response-policy option for the view. Response policy zones are ordinary DNS zones containing RRsets that can be queried normally if allowed. It is usually best to restrict those queries with something like allow-query { localhost; };. Note that zones using masterfile-format map cannot be used as policy zones.

A response-policy option can support multiple policy zones. To maximize performance, a radix tree is used to quickly identify response policy zones containing triggers that match the current query. This imposes an upper limit of 32 on the number of policy zones in a single response-policy option; more than that is a configuration error.

Rules encoded in response policy zones are processed after those defined in Access Control Lists (ACLs). All queries from clients which are not permitted access to the resolver are answered with a status code of REFUSED, regardless of configured RPZ rules.

Five policy triggers can be encoded in RPZ records.

RPZ-CLIENT-IP

IP records are triggered by the IP address of the DNS client. Client IP address triggers are encoded in records that have owner names that are subdomains of rpz-client-ip, relativized to the policy zone origin name, and encode an address or address block. IPv4 addresses are represented as prefixlength.B4.B3.B2.B1.rpz-client-ip. The IPv4 prefix length must be between 1 and 32. All four bytes - B4, B3, B2, and B1 - must be present. B4 is the decimal value of the least significant byte of the IPv4 address as in IN-ADDR.ARPA.

IPv6 addresses are encoded in a format similar to the standard IPv6 text representation, prefixlength.W8.W7.W6.W5.W4.W3.W2.W1.rpz-client-ip. Each of W8,...,W1 is a one- to four-digit hexadecimal number representing 16 bits of the IPv6 address as in the standard text representation of IPv6 addresses, but reversed as in IP6.ARPA. (Note that this representation of IPv6 address is different from IP6.ARPA where each hex digit occupies a label.) All 8 words must be present except when one set of consecutive zero words is replaced with .zz., analogous to double colons (::) in standard IPv6 text encodings. The IPv6 prefix length must be between 1 and 128.

QNAME

QNAME policy records are triggered by query names of requests and targets of CNAME records resolved to generate the response. The owner name of a QNAME policy record is the query name relativized to the policy zone.

RPZ-IP

IP triggers are IP addresses in an A or AAAA record in the ANSWER section of a response. They are encoded like client-IP triggers, except as subdomains of rpz-ip.

RPZ-NSDNAME

NSDNAME triggers match names of authoritative servers for the query name, a parent of the query name, a CNAME for the query name, or a parent of a CNAME. They are encoded as subdomains of rpz-nsdname, relativized to the RPZ origin name. NSIP triggers match IP addresses in A and AAAA RRsets for domains that can be checked against NSDNAME policy records. The nsdname-enable phrase turns NSDNAME triggers off or on for a single policy zone or for all zones.

If authoritative nameservers for the query name are not yet known, named recursively looks up the authoritative servers for the query name before applying an RPZ-NSDNAME rule, which can cause a processing delay. To speed up processing at the cost of precision, the nsdname-wait-recurse option can be used; when set to no, RPZ-NSDNAME rules are only applied when authoritative servers for the query name have already been looked up and cached. If authoritative servers for the query name are not in the cache, the RPZ-NSDNAME rule is ignored, but the authoritative servers for the query name are looked up in the background and the rule is applied to subsequent queries. The default is yes, meaning RPZ-NSDNAME rules are always applied, even if authoritative servers for the query name need to be looked up first.

RPZ-NSIP

NSIP triggers match the IP addresses of authoritative servers. They are enncoded like IP triggers, except as subdomains of rpz-nsip. NSDNAME and NSIP triggers are checked only for names with at least min-ns-dots dots. The default value of min-ns-dots is 1, to exclude top-level domains.

If a name server's IP address is not yet known, named recursively looks up the IP address before applying an RPZ-NSIP rule, which can cause a processing delay. To speed up processing at the cost of precision, the nsip-wait-recurse option can be used: when set to no, RPZ-NSIP rules are only applied when a name server's IP address has already been looked up and cached. If a server's IP address is not in the cache, the RPZ-NSIP rule is ignored, but the address is looked up in the background and the rule is applied to subsequent queries. The default is yes, meaning RPZ-NSIP rules are always applied, even if an address needs to be looked up first.

The query response is checked against all response policy zones, so two or more policy records can be triggered by a response. Because DNS responses are rewritten according to at most one policy record, a single record encoding an action (other than DISABLED actions) must be chosen. Triggers, or the records that encode them, are chosen for rewriting in the following order:

Choose the triggered record in the zone that appears first in the response-policy option.
Prefer CLIENT-IP to QNAME to IP to NSDNAME to NSIP triggers in a single zone.
Among NSDNAME triggers, prefer the trigger that matches the smallest name under the DNSSEC ordering.
Among IP or NSIP triggers, prefer the trigger with the longest prefix.
Among triggers with the same prefix length, prefer the IP or NSIP trigger that matches the smallest IP address.

When the processing of a response is restarted to resolve DNAME or CNAME records and a policy record set has not been triggered, all response policy zones are again consulted for the DNAME or CNAME names and addresses.

RPZ record sets are any types of DNS record, except DNAME or DNSSEC, that encode actions or responses to individual queries. Any of the policies can be used with any of the triggers. For example, while the TCP-only policy is commonly used with client-IP triggers, it can be used with any type of trigger to force the use of TCP for responses with owner names in a zone.

PASSTHRU

The auto-acceptance policy is specified by a CNAME whose target is rpz-passthru. It causes the response to not be rewritten and is most often used to "poke holes" in policies for CIDR blocks.

DROP

The auto-rejection policy is specified by a CNAME whose target is rpz-drop. It causes the response to be discarded. Nothing is sent to the DNS client.

TCP-Only

The "slip" policy is specified by a CNAME whose target is rpz-tcp-only. It changes UDP responses to short, truncated DNS responses that require the DNS client to try again with TCP. It is used to mitigate distributed DNS reflection attacks.

NXDOMAIN

The "domain undefined" response is encoded by a CNAME whose target is the root domain (.)

NODATA

The empty set of resource records is specified by a CNAME whose target is the wildcard top-level domain (*.). It rewrites the response to NODATA or ANCOUNT=0.

Local Data

A set of ordinary DNS records can be used to answer queries. Queries for record types not the set are answered with NODATA.

A special form of local data is a CNAME whose target is a wildcard such as *.example.com. It is used as if an ordinary CNAME after the asterisk (*) has been replaced with the query name. This special form is useful for query logging in the walled garden's authoritative DNS server.

All of the actions specified in all of the individual records in a policy zone can be overridden with a policy clause in the response-policy option. An organization using a policy zone provided by another organization might use this mechanism to redirect domains to its own walled garden.

GIVEN: The placeholder policy says "do not override but perform the action specified in the zone."
DISABLED: The testing override policy causes policy zone records to do nothing but log what they would have done if the policy zone were not disabled. The response to the DNS query is written (or not) according to any triggered policy records that are not disabled. Disabled policy zones should appear first, because they are often not logged if a higher-precedence trigger is found first.
PASSTHRU, DROP, TCP-Only, NXDOMAIN, NODATA: each override the corresponding per-record policy.
CNAME domain: causes all RPZ policy records to act as if they were "cname domain" records.

By default, the actions encoded in a response policy zone are applied only to queries that ask for recursion (RD=1). That default can be changed for a single policy zone, or for all response policy zones in a view, with a recursive-only no clause. This feature is useful for serving the same zone files both inside and outside an RFC 1918 cloud and using RPZ to delete answers that would otherwise contain RFC 1918 values on the externally visible name server or view.

Also by default, RPZ actions are applied only to DNS requests that either do not request DNSSEC metadata (DO=0) or when no DNSSEC records are available for the requested name in the original zone (not the response policy zone). This default can be changed for all response policy zones in a view with a break-dnssec yes clause. In that case, RPZ actions are applied regardless of DNSSEC. The name of the clause option reflects the fact that results rewritten by RPZ actions cannot verify.

No DNS records are needed for a QNAME or Client-IP trigger; the name or IP address itself is sufficient, so in principle the query name need not be recursively resolved. However, not resolving the requested name can leak the fact that response policy rewriting is in use, and that the name is listed in a policy zone, to operators of servers for listed names. To prevent that information leak, by default any recursion needed for a request is done before any policy triggers are considered. Because listed domains often have slow authoritative servers, this behavior can cost significant time. The qname-wait-recurse no option overrides that default behavior when recursion cannot change a non-error response. The option does not affect QNAME or client-IP triggers in policy zones listed after other zones containing IP, NSIP, and NSDNAME triggers, because those may depend on the A, AAAA, and NS records that would be found during recursive resolution. It also does not affect DNSSEC requests (DO=1) unless break-dnssec yes is in use, because the response would depend on whether RRSIG records were found during resolution. Using this option can cause error responses such as SERVFAIL to appear to be rewritten, since no recursion is being done to discover problems at the authoritative server.

The TTL of a record modified by RPZ policies is set from the TTL of the relevant record in the policy zone. It is then limited to a maximum value. The max-policy-ttl clause changes the maximum number of seconds from its default of 5.

For example, an administrator might use this option statement:

    response-policy { zone "badlist"; };

and this zone statement:

    zone "badlist" {type master; file "master/badlist"; allow-query {none;}; };

with this zone file:

$TTL 1H
@                       SOA LOCALHOST. named-mgr.example.com (1 1h 15m 30d 2h)
                        NS  LOCALHOST.

; QNAME policy records.  There are no periods (.) after the owner names.
nxdomain.domain.com     CNAME   .               ; NXDOMAIN policy
*.nxdomain.domain.com   CNAME   .               ; NXDOMAIN policy
nodata.domain.com       CNAME   *.              ; NODATA policy
*.nodata.domain.com     CNAME   *.              ; NODATA policy
bad.domain.com          A       10.0.0.1        ; redirect to a walled garden
                        AAAA    2001:2::1
bzone.domain.com        CNAME   garden.example.com.

; do not rewrite (PASSTHRU) OK.DOMAIN.COM
ok.domain.com           CNAME   rpz-passthru.

; redirect x.bzone.domain.com to x.bzone.domain.com.garden.example.com
*.bzone.domain.com      CNAME   *.garden.example.com.


; IP policy records that rewrite all responses containing A records in 127/8
;       except 127.0.0.1
8.0.0.0.127.rpz-ip      CNAME   .
32.1.0.0.127.rpz-ip     CNAME   rpz-passthru.

; NSDNAME and NSIP policy records
ns.domain.com.rpz-nsdname   CNAME   .
48.zz.2.2001.rpz-nsip       CNAME   .

; auto-reject and auto-accept some DNS clients
112.zz.2001.rpz-client-ip    CNAME   rpz-drop.
8.0.0.0.127.rpz-client-ip    CNAME   rpz-drop.

; force some DNS clients and responses in the example.com zone to TCP
16.0.0.1.10.rpz-client-ip   CNAME   rpz-tcp-only.
example.com                 CNAME   rpz-tcp-only.
*.example.com               CNAME   rpz-tcp-only.

RPZ can affect server performance. Each configured response policy zone requires the server to perform one to four additional database lookups before a query can be answered. For example, a DNS server with four policy zones, each with all four kinds of response triggers (QNAME, IP, NSIP, and NSDNAME), requires a total of 17 times as many database lookups as a similar DNS server with no response policy zones. A BIND 9 server with adequate memory and one response policy zone with QNAME and IP triggers might achieve a maximum queries-per-second (QPS) rate about 20% lower. A server with four response policy zones with QNAME and IP triggers might have a maximum QPS rate about 50% lower.

Responses rewritten by RPZ are counted in the RPZRewrites statistics.

The log clause can be used to optionally turn off rewrite logging for a particular response policy zone. By default, all rewrites are logged.

Response Rate Limiting

Excessive, almost identical UDP responses can be controlled by configuring a rate-limit clause in an options or view statement. This mechanism keeps authoritative BIND 9 from being used to amplify reflection denial of service (DoS) attacks. Short, truncated (TC=1) responses can be sent to provide rate-limited responses to legitimate clients within a range of forged, attacked IP addresses. Legitimate clients react to dropped or truncated responses by retrying with UDP or with TCP, respectively.

This mechanism is intended for authoritative DNS servers. It can be used on recursive servers, but can slow applications such as SMTP servers (mail receivers) and HTTP clients (web browsers) that repeatedly request the same domains. When possible, closing "open" recursive servers is better.

Response rate limiting uses a "credit" or "token bucket" scheme. Each combination of identical response and client has a conceptual "account" that earns a specified number of credits every second. A prospective response debits its account by one. Responses are dropped or truncated while the account is negative. Responses are tracked within a rolling window of time which defaults to 15 seconds, but which can be configured with the window option to any value from 1 to 3600 seconds (1 hour). The account cannot become more positive than the per-second limit or more negative than window times the per-second limit. When the specified number of credits for a class of responses is set to 0, those responses are not rate-limited.

The notions of "identical response" and "DNS client" for rate limiting are not simplistic. All responses to an address block are counted as if to a single client. The prefix lengths of address blocks are specified with ipv4-prefix-length (default 24) and ipv6-prefix-length (default 56).

All non-empty responses for a valid domain name (qname) and record type (qtype) are identical and have a limit specified with responses-per-second (default 0 or no limit). All empty (NODATA) responses for a valid domain, regardless of query type, are identical. Responses in the NODATA class are limited by nodata-per-second (default responses-per-second). Requests for any and all undefined subdomains of a given valid domain result in NXDOMAIN errors, and are identical regardless of query type. They are limited by nxdomains-per-second (default responses-per-second). This controls some attacks using random names, but can be relaxed or turned off (set to 0) on servers that expect many legitimate NXDOMAIN responses, such as from anti-spam rejection lists. Referrals or delegations to the server of a given domain are identical and are limited by referrals-per-second (default responses-per-second).

Responses generated from local wildcards are counted and limited as if they were for the parent domain name. This controls flooding using random.wild.example.com.

All requests that result in DNS errors other than NXDOMAIN, such as SERVFAIL and FORMERR, are identical regardless of requested name (qname) or record type (qtype). This controls attacks using invalid requests or distant, broken authoritative servers. By default the limit on errors is the same as the responses-per-second value, but it can be set separately with errors-per-second.

Many attacks using DNS involve UDP requests with forged source addresses. Rate limiting prevents the use of BIND 9 to flood a network with responses to requests with forged source addresses, but could let a third party block responses to legitimate requests. There is a mechanism that can answer some legitimate requests from a client whose address is being forged in a flood. Setting slip to 2 (its default) causes every other UDP request to be answered with a small truncated (TC=1) response. The small size and reduced frequency, and resulting lack of amplification, of "slipped" responses make them unattractive for reflection DoS attacks. slip must be between 0 and 10. A value of 0 does not "slip"; no truncated responses are sent due to rate limiting. Rather, all responses are dropped. A value of 1 causes every response to slip; values between 2 and 10 cause every nth response to slip. Some error responses, including REFUSED and SERVFAIL, cannot be replaced with truncated responses and are instead leaked at the slip rate.

(Note: dropped responses from an authoritative server may reduce the difficulty of a third party successfully forging a response to a recursive resolver. The best security against forged responses is for authoritative operators to sign their zones using DNSSEC and for resolver operators to validate the responses. When this is not an option, operators who are more concerned with response integrity than with flood mitigation may consider setting slip to 1, causing all rate-limited responses to be truncated rather than dropped. This reduces the effectiveness of rate-limiting against reflection attacks.)

When the approximate query-per-second rate exceeds the qps-scale value, the responses-per-second, errors-per-second, nxdomains-per-second, and all-per-second values are reduced by the ratio of the current rate to the qps-scale value. This feature can tighten defenses during attacks. For example, with qps-scale 250; responses-per-second 20; and a total query rate of 1000 queries/second for all queries from all DNS clients including via TCP, then the effective responses/second limit changes to (250/1000)*20, or 5. Responses sent via TCP are not limited but are counted to compute the query-per-second rate.

Communities of DNS clients can be given their own parameters or no rate limiting by putting rate-limit statements in view statements instead of in the global option statement. A rate-limit statement in a view replaces, rather than supplements, a rate-limit statement among the main options. DNS clients within a view can be exempted from rate limits with the exempt-clients clause.

UDP responses of all kinds can be limited with the all-per-second phrase. This rate limiting is unlike the rate limiting provided by responses-per-second, errors-per-second, and nxdomains-per-second on a DNS server, which are often invisible to the victim of a DNS reflection attack. Unless the forged requests of the attack are the same as the legitimate requests of the victim, the victim's requests are not affected. Responses affected by an all-per-second limit are always dropped; the slip value has no effect. An all-per-second limit should be at least 4 times as large as the other limits, because single DNS clients often send bursts of legitimate requests. For example, the receipt of a single mail message can prompt requests from an SMTP server for NS, PTR, A, and AAAA records as the incoming SMTP/TCP/IP connection is considered. The SMTP server can need additional NS, A, AAAA, MX, TXT, and SPF records as it considers the SMTP Mail From command. Web browsers often repeatedly resolve the same names that are duplicated in HTML <IMG> tags in a page. all-per-second is similar to the rate limiting offered by firewalls but is often inferior. Attacks that justify ignoring the contents of DNS responses are likely to be attacks on the DNS server itself. They usually should be discarded before the DNS server spends resources make TCP connections or parsing DNS requests, but that rate limiting must be done before the DNS server sees the requests.

The maximum size of the table used to track requests and rate-limit responses is set with max-table-size. Each entry in the table is between 40 and 80 bytes. The table needs approximately as many entries as the number of requests received per second. The default is 20,000. To reduce the cold start of growing the table, min-table-size (default 500) can set the minimum table size. Enable rate-limit category logging to monitor expansions of the table and inform choices for the initial and maximum table size.

Use log-only yes to test rate-limiting parameters without actually dropping any requests.

Responses dropped by rate limits are included in the RateDropped and QryDropped statistics. Responses that truncated by rate limits are included in RateSlipped and RespTruncated.

NXDOMAIN Redirection

named supports NXDOMAIN redirection via two methods:

Redirect zone the section called “zone Statement Grammar”
Redirect namespace

With either method, when named gets an NXDOMAIN response it examines a separate namespace to see if the NXDOMAIN response should be replaced with an alternative response.

With a redirect zone (zone "." { type redirect; };), the data used to replace the NXDOMAIN is held in a single zone which is not part of the normal namespace. All the redirect information is contained in the zone; there are no delegations.

With a redirect namespace (option { nxdomain-redirect <suffix> };), the data used to replace the NXDOMAIN is part of the normal namespace and is looked up by appending the specified suffix to the original query name. This roughly doubles the cache required to process NXDOMAIN responses, as both the original NXDOMAIN response and the replacement data (or a NXDOMAIN indicating that there is no replacement) must be stored.

If both a redirect zone and a redirect namespace are configured, the redirect zone is tried first.

server Statement Grammar

server netprefix {
	bogus boolean;
	edns boolean;
	edns-udp-size integer;
	edns-version integer;
	keys server_key;
	max-udp-size integer;
	notify-source ( ipv4_address | * ) [ port ( integer | * ) ] [
	    dscp integer ];
	notify-source-v6 ( ipv6_address | * ) [ port ( integer | * ) ]
	    [ dscp integer ];
	provide-ixfr boolean;
	query-source ( ( [ address ] ( ipv4_address | * ) [ port (
	    integer | * ) ] ) | ( [ [ address ] ( ipv4_address | * ) ]
	    port ( integer | * ) ) ) [ dscp integer ];
	query-source-v6 ( ( [ address ] ( ipv6_address | * ) [ port (
	    integer | * ) ] ) | ( [ [ address ] ( ipv6_address | * ) ]
	    port ( integer | * ) ) ) [ dscp integer ];
	request-expire boolean;
	request-ixfr boolean;
	request-nsid boolean;
	send-cookie boolean;
	tcp-only boolean;
	transfer-format ( many-answers | one-answer );
	transfer-source ( ipv4_address | * ) [ port ( integer | * ) ] [
	    dscp integer ];
	transfer-source-v6 ( ipv6_address | * ) [ port ( integer | * )
	    ] [ dscp integer ];
	transfers integer;
};

server Statement Definition and Usage

The server statement defines characteristics to be associated with a remote name server. If a prefix length is specified, then a range of servers is covered. Only the most specific server clause applies, regardless of the order in named.conf.

The server statement can occur at the top level of the configuration file or inside a view statement. If a view statement contains one or more server statements, only those apply to the view and any top-level ones are ignored. If a view contains no server statements, any top-level server statements are used as defaults.

If a remote server is giving out bad data, marking it as bogus prevents further queries to it. The default value of bogus is no.

The provide-ixfr clause determines whether the local server, acting as primary, responds with an incremental zone transfer when the given remote server, a secondary, requests it. If set to yes, incremental transfer is provided whenever possible. If set to no, all transfers to the remote server are non-incremental. If not set, the value of the provide-ixfr option in the view or global options block is used as a default.

The request-ixfr clause determines whether the local server, acting as a secondary, requests incremental zone transfers from the given remote server, a primary. If not set, the value of the request-ixfr option in the view or global options block is used as a default. It may also be set in the zone block; if set there, it overrides the global or view setting for that zone.

IXFR requests to servers that do not support IXFR automatically fall back to AXFR. Therefore, there is no need to manually list which servers support IXFR and which ones do not; the global default of yes should always work. The purpose of the provide-ixfr and request-ixfr clauses is to make it possible to disable the use of IXFR even when both primary and secondary claim to support it: for example, if one of the servers is buggy and crashes or corrupts data when IXFR is used.

The request-expire clause determines whether the local server, when acting as a secondary, requests the EDNS EXPIRE value. The EDNS EXPIRE value indicates the remaining time before the zone data expires and needs to be refreshed. This is used when a secondary server transfers a zone from another secondary server; when transferring from the primary, the expiration timer is set from the EXPIRE field of the SOA record instead. The default is yes.

The edns clause determines whether the local server attempts to use EDNS when communicating with the remote server. The default is yes.

The edns-udp-size option sets the EDNS UDP size that is advertised by named when querying the remote server. Valid values are 512 to 4096 bytes; values outside this range are silently adjusted to the nearest value within it. This option is useful when advertising a different value to this server than the value advertised globally: for example, when there is a firewall at the remote site that is blocking large replies. Note: currently, this sets a single UDP size for all packets sent to the server; named does not deviate from this value. This differs from the behavior of edns-udp-size in options or view statements, where it specifies a maximum value. The server statement behavior may be brought into conformance with the options/view behavior in future releases.

The edns-version option sets the maximum EDNS VERSION that is sent to the server(s) by the resolver. The actual EDNS version sent is still subject to normal EDNS version-negotiation rules (see RFC 6891), the maximum EDNS version supported by the server, and any other heuristics that indicate that a lower version should be sent. This option is intended to be used when a remote server reacts badly to a given EDNS version or higher; it should be set to the highest version the remote server is known to support. Valid values are 0 to 255; higher values are silently adjusted. This option is not needed until higher EDNS versions than 0 are in use.

The max-udp-size option sets the maximum EDNS UDP message size named sends. Valid values are 512 to 4096 bytes; values outside this range are silently adjusted. This option is useful when there is a firewall that is blocking large replies from named.

The tcp-only option sets the transport protocol to TCP. The default is to use the UDP transport and to fallback on TCP only when a truncated response is received.

The server supports two zone transfer methods. The first, one-answer, uses one DNS message per resource record transferred. many-answers packs as many resource records as possible into a single message, which is more efficient. It is possible to specify which method to use for a server via the transfer-format option; If not set there, the transfer-format specified by the options statement is used.

transfers is used to limit the number of concurrent inbound zone transfers from the specified server. If no transfers clause is specified, the limit is set according to the transfers-per-ns option.

The keys clause identifies a key_id defined by the key statement, to be used for transaction security (TSIG, the section called “TSIG”) when talking to the remote server. When a request is sent to the remote server, a request signature is generated using the key specified here and appended to the message. A request originating from the remote server is not required to be signed by this key.

Only a single key per server is currently supported.

The transfer-source and transfer-source-v6 clauses specify the IPv4 and IPv6 source address, respectively, to be used for zone transfer with the remote server. For an IPv4 remote server, only transfer-source can be specified. Similarly, for an IPv6 remote server, only transfer-source-v6 can be specified. For more details, see the description of transfer-source and transfer-source-v6 in the section called “Zone Transfers”.

The notify-source and notify-source-v6 clauses specify the IPv4 and IPv6 source address, respectively, to be used for notify messages sent to remote servers. For an IPv4 remote server, only notify-source can be specified. Similarly, for an IPv6 remote server, only notify-source-v6 can be specified.

The query-source and query-source-v6 clauses specify the IPv4 and IPv6 source address, respectively, to be used for queries sent to remote servers. For an IPv4 remote server, only query-source can be specified. Similarly, for an IPv6 remote server, only query-source-v6 can be specified.

The request-nsid clause determines whether the local server adds an NSID EDNS option to requests sent to the server. This overrides request-nsid set at the view or option level.

The send-cookie clause determines whether the local server adds a COOKIE EDNS option to requests sent to the server. This overrides send-cookie set at the view or option level. The named server may determine that COOKIE is not supported by the remote server and not add a COOKIE EDNS option to requests.

statistics-channels Statement Grammar

statistics-channels {
	inet ( ipv4_address | ipv6_address |
	    * ) [ port ( integer | * ) ] [
	    allow { address_match_element; ...
	    } ];
};

statistics-channels Statement Definition and Usage

The statistics-channels statement declares communication channels to be used by system administrators to get access to statistics information on the name server.

This statement is intended to be flexible to support multiple communication protocols in the future, but currently only HTTP access is supported. It requires that BIND 9 be compiled with libxml2 and/or json-c (also known as libjson0); the statistics-channels statement is still accepted even if it is built without the library, but any HTTP access fails with an error.

If no port is specified, port 80 is used for HTTP channels. The asterisk (*) cannot be used for ip_port.

Attempts to open a statistics channel are restricted by the optional allow clause. Connections to the statistics channel are permitted based on the address_match_list. If no allow clause is present, named accepts connection attempts from any address; since the statistics may contain sensitive internal information, it is highly recommended to restrict the source of connection requests appropriately.

If no statistics-channels statement is present, named does not open any communication channels.

The statistics are available in various formats and views, depending on the URI used to access them. For example, if the statistics channel is configured to listen on 127.0.0.1 port 8888, then the statistics are accessible in XML format at http://127.0.0.1:8888/ or http://127.0.0.1:8888/xml. A CSS file is included, which can format the XML statistics into tables when viewed with a stylesheet-capable browser, and into charts and graphs using the Google Charts API when using a JavaScript-capable browser.

Broken-out subsets of the statistics can be viewed at http://127.0.0.1:8888/xml/v3/status (server uptime and last reconfiguration time), http://127.0.0.1:8888/xml/v3/server (server and resolver statistics), http://127.0.0.1:8888/xml/v3/zones (zone statistics), http://127.0.0.1:8888/xml/v3/net (network status and socket statistics), http://127.0.0.1:8888/xml/v3/mem (memory manager statistics), http://127.0.0.1:8888/xml/v3/tasks (task manager statistics), and http://127.0.0.1:8888/xml/v3/traffic (traffic sizes).

The full set of statistics can also be read in JSON format at http://127.0.0.1:8888/json, with the broken-out subsets at http://127.0.0.1:8888/json/v1/status (server uptime and last reconfiguration time), http://127.0.0.1:8888/json/v1/server (server and resolver statistics), http://127.0.0.1:8888/json/v1/zones (zone statistics), http://127.0.0.1:8888/json/v1/net (network status and socket statistics), http://127.0.0.1:8888/json/v1/mem (memory manager statistics), http://127.0.0.1:8888/json/v1/tasks (task manager statistics), and http://127.0.0.1:8888/json/v1/traffic (traffic sizes).

trusted-keys Statement Grammar

trusted-keys { string integer integer
    integer quoted_string; ... };

trusted-keys Statement Definition and Usage

The trusted-keys statement defines DNSSEC security roots. DNSSEC is described in the section called “DNSSEC”. A security root is defined when the public key for a non-authoritative zone is known, but cannot be securely obtained through DNS, either because it is the DNS root zone or because its parent zone is unsigned. Once a key has been configured as a trusted key, it is treated as if it has been validated and proven secure. The resolver attempts DNSSEC validation on all DNS data in subdomains of a security root.

All keys (and corresponding zones) listed in trusted-keys are deemed to exist regardless of what parent zones say. Similarly, for all keys listed in trusted-keys, only those keys are used to validate the DNSKEY RRset. The parent's DS RRset is not used.

The trusted-keys statement can contain multiple key entries, each consisting of the key's domain name, flags, protocol, and algorithm, and the Base64 representation of the key data. Spaces, tabs, newlines, and carriage returns are ignored in the key data, so the configuration may be split into multiple lines.

trusted-keys may be set at the top level of named.conf or within a view. If it is set in both places, they are additive; keys defined at the top level are inherited by all views, but keys defined in a view are only used within that view.

Validation below specified names can be temporarily disabled by using rndc nta.

managed-keys Statement Grammar

managed-keys { string string integer
    integer integer quoted_string; ... };

managed-keys Statement Definition and Usage

The managed-keys statement, like trusted-keys, defines DNSSEC security roots. The difference is that managed-keys can be kept up-to-date automatically, without intervention from the resolver operator.

Suppose, for example, that a zone's key-signing key was compromised, and the zone owner had to revoke and replace the key. A resolver which had the old key in a trusted-keys statement would be unable to validate this zone; it would reply with a SERVFAIL response code. This would continue until the resolver operator updated the trusted-keys statement with the new key.

If, however, the zone were listed in a managed-keys statement instead, the zone owner could add a "stand-by" key to the zone in advance. named would store the stand-by key, and when the original key was revoked, named would be able to transition smoothly to the new key. It would also recognize that the old key had been revoked and cease using that key to validate answers, minimizing the damage that the compromised key could do.

A managed-keys statement contains a list of the keys to be managed, along with information about how the keys are to be initialized for the first time. The only initialization method currently supported is initial-key. This means the managed-keys statement must contain a copy of the initializing key. (Future releases may allow keys to be initialized by other methods, eliminating this requirement.)

Consequently, a managed-keys statement appears similar to a trusted-keys statement, differing by the presence of the second field, which contains the keyword initial-key. The difference is, whereas the keys listed in a trusted-keys continue to be trusted until they are removed from named.conf, an initializing key listed in a managed-keys statement is only trusted once: for as long as it takes to load the managed-key database and start the RFC 5011 key-maintenance process.

The first time named runs with a managed key configured in named.conf, it fetches the DNSKEY RRset directly from the zone apex, and validates it using the key specified in the managed-keys statement. If the DNSKEY RRset is validly signed, then it is used as the basis for a new managed-keys database.

From that point on, whenever named runs, it sees the managed-keys statement, checks to make sure RFC 5011 key maintenance has already been initialized for the specified domain, and if so, simply moves on. The key specified in the managed-keys statement is not used to validate answers; it is superseded by the key or keys stored in the managed-keys database.

The next time named runs after a name has been removed from the managed-keys statement, the corresponding zone is removed from the managed-keys database, and RFC 5011 key maintenance is no longer used for that domain.

In the current implementation, the managed-keys database is stored as a master-format zone file.

On servers which do not use views, this file is named managed-keys.bind. When views are in use, there is a separate managed-keys database for each view; the filename is the view name (or, if a view name contains characters which would make it illegal as a filename, a hash of the view name), followed by the suffix .mkeys.

When the key database is changed, the zone is updated. As with any other dynamic zone, changes are written into a journal file, e.g., managed-keys.bind.jnl or internal.mkeys.jnl. Changes are committed to the zone file as soon as possible afterward, usually within 30 seconds. Whenever named is using automatic key maintenance, the zone file and journal file can be expected to exist in the working directory. (For this reason, among others, the working directory should be always be writable by named.)

If the dnssec-validation option is set to auto, named automatically initializes a managed key for the root zone. The key that is used to initialize the key-maintenance process is stored in bind.keys; the location of this file can be overridden with the bindkeys-file option. As a fallback in the event no bind.keys can be found, the initializing key is also compiled directly into named.

view Statement Grammar

view view_name [ class ] {
    match-clients { address_match_list } ;
    match-destinations { address_match_list } ;
    match-recursive-only yes_or_no ;
  [ view_option ; ... ]
  [ zone_statement ; ... ]
} ;

view Statement Definition and Usage

The view statement is a powerful feature of BIND 9 that lets a name server answer a DNS query differently depending on who is asking. It is particularly useful for implementing split DNS setups without having to run multiple servers.

Each view statement defines a view of the DNS namespace that is seen by a subset of clients. A client matches a view if its source IP address matches the address_match_list of the view's match-clients clause and its destination IP address matches the address_match_list of the view's match-destinations clause. If not specified, both match-clients and match-destinations default to matching all addresses. In addition to checking IP addresses, match-clients and match-destinations can also take keys which provide an mechanism for the client to select the view. A view can also be specified as match-recursive-only, which means that only recursive requests from matching clients match that view. The order of the view statements is significant; a client request is resolved in the context of the first view that it matches.

Zones defined within a view statement are only accessible to clients that match the view. By defining a zone of the same name in multiple views, different zone data can be given to different clients: for example, "internal" and "external" clients in a split DNS setup.

Many of the options given in the options statement can also be used within a view statement, and then apply only when resolving queries with that view. When no view-specific value is given, the value in the options statement is used as a default. Also, zone options can have default values specified in the view statement; these view-specific defaults take precedence over those in the options statement.

Views are class-specific. If no class is given, class IN is assumed. Note that all non-IN views must contain a hint zone, since only the IN class has compiled-in default hints.

If there are no view statements in the config file, a default view that matches any client is automatically created in class IN. Any zone statements specified on the top level of the configuration file are considered to be part of this default view, and the options statement applies to the default view. If any explicit view statements are present, all zone statements must occur inside view statements.

Here is an example of a typical split DNS setup implemented using view statements:

view "internal" {
      // This should match our internal networks.
      match-clients { 10.0.0.0/8; };

      // Provide recursive service to internal
      // clients only.
      recursion yes;

      // Provide a complete view of the example.com
      // zone including addresses of internal hosts.
      zone "example.com" {
            type master;
            file "example-internal.db";
      };
};

view "external" {
      // Match all clients not matched by the
      // previous view.
      match-clients { any; };

      // Refuse recursive service to external clients.
      recursion no;

      // Provide a restricted view of the example.com
      // zone containing only publicly accessible hosts.
      zone "example.com" {
           type master;
           file "example-external.db";
      };
};

zone Statement Grammar

zone string [ class ] {
	type ( master | primary );
	allow-query { address_match_element; ... };
	allow-query-on { address_match_element; ... };
	allow-transfer { address_match_element; ... };
	allow-update { address_match_element; ... };
	also-notify [ port integer ] [ dscp integer ] { ( masters | ipv4_address [ port integer ] | ipv6_address [ port integer ] ) [ key string ]; ... };
	alt-transfer-source ( ipv4_address | * ) [ port ( integer | * ) ] [ dscp integer ];
	alt-transfer-source-v6 ( ipv6_address | * ) [ port ( integer | * ) ] [ dscp integer ];
	auto-dnssec ( allow | maintain | off );
	check-dup-records ( fail | warn | ignore );
	check-integrity boolean;
	check-mx ( fail | warn | ignore );
	check-mx-cname ( fail | warn | ignore );
	check-names ( fail | warn | ignore );
	check-sibling boolean;
	check-spf ( warn | ignore );
	check-srv-cname ( fail | warn | ignore );
	check-wildcard boolean;
	database string;
	dialup ( notify | notify-passive | passive | refresh | boolean );
	dlz string;
	dnssec-dnskey-kskonly boolean;
	dnssec-loadkeys-interval integer;
	dnssec-secure-to-insecure boolean;
	dnssec-update-mode ( maintain | no-resign );
	file quoted_string;
	forward ( first | only );
	forwarders [ port integer ] [ dscp integer ] { ( ipv4_address | ipv6_address ) [ port integer ] [ dscp integer ]; ... };
	inline-signing boolean;
	ixfr-from-differences boolean;
	journal quoted_string;
	key-directory quoted_string;
	masterfile-format ( map | raw | text );
	masterfile-style ( full | relative );
	max-journal-size ( unlimited | sizeval );
	max-records integer;
	max-transfer-idle-out integer;
	max-transfer-time-out integer;
	max-zone-ttl ( unlimited | ttlval );
	notify ( explicit | master-only | boolean );
	notify-delay integer;
	notify-source ( ipv4_address | * ) [ port ( integer | * ) ] [ dscp integer ];
	notify-source-v6 ( ipv6_address | * ) [ port ( integer | * ) ] [ dscp integer ];
	notify-to-soa boolean;
	serial-update-method ( date | increment | unixtime );
	sig-signing-nodes integer;
	sig-signing-signatures integer;
	sig-signing-type integer;
	sig-validity-interval integer [ integer ];
	update-check-ksk boolean;
	update-policy ( local | { ( deny | grant ) string ( 6to4-self | external | krb5-self | krb5-selfsub | krb5-subdomain | ms-self | ms-selfsub | ms-subdomain | name | self | selfsub | selfwild | subdomain | tcp-self | wildcard | zonesub ) [ string ] rrtypelist; ... };
	zero-no-soa-ttl boolean;
	zone-statistics ( full | terse | none | boolean );
};

zone string [ class ] {
	type ( slave | secondary );
	allow-notify { address_match_element; ... };
	allow-query { address_match_element; ... };
	allow-query-on { address_match_element; ... };
	allow-transfer { address_match_element; ... };
	allow-update-forwarding { address_match_element; ... };
	also-notify [ port integer ] [ dscp integer ] { ( masters | ipv4_address [ port integer ] | ipv6_address [ port integer ] ) [ key string ]; ... };
	alt-transfer-source ( ipv4_address | * ) [ port ( integer | * ) ] [ dscp integer ];
	alt-transfer-source-v6 ( ipv6_address | * ) [ port ( integer | * ) ] [ dscp integer ];
	auto-dnssec ( allow | maintain | off );
	check-names ( fail | warn | ignore );
	database string;
	dialup ( notify | notify-passive | passive | refresh | boolean );
	dlz string;
	dnssec-dnskey-kskonly boolean;
	dnssec-loadkeys-interval integer;
	dnssec-update-mode ( maintain | no-resign );
	file quoted_string;
	forward ( first | only );
	forwarders [ port integer ] [ dscp integer ] { ( ipv4_address | ipv6_address ) [ port integer ] [ dscp integer ]; ... };
	inline-signing boolean;
	ixfr-from-differences boolean;
	journal quoted_string;
	key-directory quoted_string;
	masterfile-format ( map | raw | text );
	masterfile-style ( full | relative );
	masters [ port integer ] [ dscp integer ] { ( masters | ipv4_address [ port integer ] | ipv6_address [ port integer ] ) [ key string ]; ... };
	max-journal-size ( unlimited | sizeval );
	max-records integer;
	max-refresh-time integer;
	max-retry-time integer;
	max-transfer-idle-in integer;
	max-transfer-idle-out integer;
	max-transfer-time-in integer;
	max-transfer-time-out integer;
	min-refresh-time integer;
	min-retry-time integer;
	multi-master boolean;
	notify ( explicit | master-only | boolean );
	notify-delay integer;
	notify-source ( ipv4_address | * ) [ port ( integer | * ) ] [ dscp integer ];
	notify-source-v6 ( ipv6_address | * ) [ port ( integer | * ) ] [ dscp integer ];
	notify-to-soa boolean;
	request-expire boolean;
	request-ixfr boolean;
	sig-signing-nodes integer;
	sig-signing-signatures integer;
	sig-signing-type integer;
	sig-validity-interval integer [ integer ];
	transfer-source ( ipv4_address | * ) [ port ( integer | * ) ] [ dscp integer ];
	transfer-source-v6 ( ipv6_address | * ) [ port ( integer | * ) ] [ dscp integer ];
	try-tcp-refresh boolean;
	update-check-ksk boolean;
	use-alt-transfer-source boolean;
	zero-no-soa-ttl boolean;
	zone-statistics ( full | terse | none | boolean );
};

zone string [ class ] {
	type hint;
	check-names ( fail | warn | ignore );
	delegation-only boolean;
	file quoted_string;
};

zone string [ class ] {
	type stub;
	allow-query { address_match_element; ... };
	allow-query-on { address_match_element; ... };
	check-names ( fail | warn | ignore );
	database string;
	delegation-only boolean;
	dialup ( notify | notify-passive | passive | refresh | boolean );
	file quoted_string;
	forward ( first | only );
	forwarders [ port integer ] [ dscp integer ] { ( ipv4_address | ipv6_address ) [ port integer ] [ dscp integer ]; ... };
	masterfile-format ( map | raw | text );
	masterfile-style ( full | relative );
	masters [ port integer ] [ dscp integer ] { ( masters | ipv4_address [ port integer ] | ipv6_address [ port integer ] ) [ key string ]; ... };
	max-records integer;
	max-refresh-time integer;
	max-retry-time integer;
	max-transfer-idle-in integer;
	max-transfer-time-in integer;
	min-refresh-time integer;
	min-retry-time integer;
	multi-master boolean;
	transfer-source ( ipv4_address | * ) [ port ( integer | * ) ] [ dscp integer ];
	transfer-source-v6 ( ipv6_address | * ) [ port ( integer | * ) ] [ dscp integer ];
	use-alt-transfer-source boolean;
	zone-statistics ( full | terse | none | boolean );
};

zone string [ class ] {
	type static-stub;
	allow-query { address_match_element; ... };
	allow-query-on { address_match_element; ... };
	forward ( first | only );
	forwarders [ port integer ] [ dscp integer ] { ( ipv4_address | ipv6_address ) [ port integer ] [ dscp integer ]; ... };
	max-records integer;
	server-addresses { ( ipv4_address | ipv6_address ); ... };
	server-names { quoted_string; ... };
	zone-statistics ( full | terse | none | boolean );
};

zone string [ class ] {
	type forward;
	delegation-only boolean;
	forward ( first | only );
	forwarders [ port integer ] [ dscp integer ] { ( ipv4_address | ipv6_address ) [ port integer ] [ dscp integer ]; ... };
};

zone string [ class ] {
	type redirect;
	allow-query { address_match_element; ... };
	allow-query-on { address_match_element; ... };
	dlz string;
	file quoted_string;
	masterfile-format ( map | raw | text );
	masterfile-style ( full | relative );
	masters [ port integer ] [ dscp integer ] { ( masters | ipv4_address [ port integer ] | ipv6_address [ port integer ] ) [ key string ]; ... };
	max-records integer;
	max-zone-ttl ( unlimited | ttlval );
	zone-statistics ( full | terse | none | boolean );
};

zone string [ class ] {
	type delegation-only;
};

zone string [ class ] {
	in-view string;
};

zone Statement Definition and Usage

Zone Types

The type keyword is required for the zone configuration unless it is an in-view configuration. Its acceptable values are: master, slave, hint, stub, static-stub, forward, redirect, or delegation-only.

Note

Later versions of BIND added type primary and type secondary as synonyms for type master and type slave, as those terms are in more common use now. BIND 9.11's configuration syntax predates this change.

`master`	The server has a master copy of the data for the zone and is able to provide authoritative answers for it.
`slave`	A secondary zone, replicating a primary zone provided by another authoritative server. The masters list specifies one or more IP addresses of primary servers that the secondary contacts to update its copy of the zone. Masters list elements can also be names of other masters lists. By default, transfers are made from port 53 on the servers; this can be changed for all servers by specifying a port number before the list of IP addresses, or on a per-server basis after the IP address. Authentication to the primary can also be done with per-server TSIG keys. If a file is specified, then the replica is written to this file whenever the zone is changed, and reloaded from this file on a server restart. Use of a file is recommended, since it often speeds server startup and eliminates a needless waste of bandwidth. Note that for large numbers (in the tens or hundreds of thousands) of zones per server, it is best to use a two-level naming scheme for zone filenames. For example, a secondary server for the zone `example.com` might place the zone contents into a file called `ex/example.com` where `ex/` is just the first two letters of the zone name. (Most operating systems behave very slowly if there are 100000 files in a single directory.)
`hint`	The initial set of root name servers is specified using a hint zone. When the server starts, it uses the root hints to find a root name server and get the most recent list of root name servers. If no hint zone is specified for class IN, the server uses a compiled-in default set of root servers hints. Classes other than IN have no built-in default hints.
`stub`	A stub zone is similar to a secondary zone, except that it replicates only the NS records of a primary zone instead of the entire zone. Stub zones are not a standard part of the DNS; they are a feature specific to the BIND implementation. Stub zones can be used to eliminate the need for a glue NS record in a parent zone, at the expense of maintaining a stub zone entry and a set of name server addresses in `named.conf`. This usage is not recommended for new configurations, and BIND 9 supports it only in a limited way. If a BIND 9 primary, serving a parent zone, has child stub zones configured, all the secondary servers for the parent zone also need to have the same child stub zones configured. Stub zones can also be used as a way to force the resolution of a given domain to use a particular set of authoritative servers. For example, the caching name servers on a private network using RFC 1918 addressing may be configured with stub zones for `10.in-addr.arpa` to use a set of internal name servers as the authoritative servers for that domain.
`static-stub`	A static-stub zone is similar to a stub zone with the following exceptions: the zone data is statically configured, rather than transferred from a primary server; and when recursion is necessary for a query that matches a static-stub zone, the locally configured data (name server names and glue addresses) is always used, even if different authoritative information is cached. Zone data is configured via the server-addresses and server-names zone options. The zone data is maintained in the form of NS and (if necessary) glue A or AAAA RRs internally, which can be seen by dumping zone databases by rndc dumpdb -all. The configured RRs are considered local configuration parameters rather than public data. Non-recursive queries (i.e., those with the RD bit off) to a static-stub zone are therefore prohibited and are responded to with REFUSED. Since the data is statically configured, no zone maintenance action takes place for a static-stub zone. For example, there is no periodic refresh attempt, and an incoming notify message will be rejected with an rcode of NOTAUTH. Each static-stub zone is configured with internally generated NS and (if necessary) glue A or AAAA RRs.
`forward`	A forward zone is a way to configure forwarding on a per-domain basis. A zone statement of type forward can contain a forward and/or forwarders statement, which applies to queries within the domain given by the zone name. If no forwarders statement is present, or an empty list for forwarders is given, then no forwarding is done for the domain, canceling the effects of any forwarders in the options statement. Thus, to use this type of zone to change the behavior of the global forward option (that is, "forward first" to, then "forward only", or vice versa), but use the same servers as set globally, re-specify the global forwarders.
`redirect`	Redirect zones are used to provide answers to queries when normal resolution would result in NXDOMAIN being returned. Only one redirect zone is supported per view. allow-query can be used to restrict which clients see these answers. If the client has requested DNSSEC records (DO=1) and the NXDOMAIN response is signed, no substitution occurs. To redirect all NXDOMAIN responses to 100.100.100.2 and 2001:ffff:ffff::100.100.100.2, configure a type `redirect` zone named ".", with the zone file containing wildcard records that point to the desired addresses: `. IN A 100.100.100.2` and `. IN AAAA 2001:ffff:ffff::100.100.100.2`. As another example, to redirect all Spanish names (under .ES), use similar entries but with the names ".ES." instead of ".". To redirect all commercial Spanish names (under COM.ES), use wildcard entries called ".COM.ES.". Note that the redirect zone supports all possible types; it is not limited to A and AAAA records. Because redirect zones are not referenced directly by name, they are not kept in the zone lookup table with normal primary and secondary zones. Consequently, it is not currently possible to use rndc reload `zonename`* to reload a redirect zone. However, when using rndc reload without specifying a zone name, redirect zones are reloaded along with other zones.
`delegation-only`	This zone type is used to enforce the delegation-only status of infrastructure zones (e.g., COM, NET, ORG). Any answer that is received without an explicit or implicit delegation in the authority section is treated as NXDOMAIN. This does not apply to the zone apex, and should not be applied to leaf zones. `delegation-only` has no effect on answers received from forwarders. See caveats in root-delegation-only.
`in-view`	When using multiple views, a primary or secondary zone configured in one view can be referenced in a subsequent view. This allows both views to serve the same zone without the overhead of loading it more than once. This is configured using a `zone` statement, with an `in-view` option specifying the view in which the zone is defined. A `zone` statement containing `in-view` does not need to specify a type, since that is part of the zone definition in the other view. See the section called “Multiple Views” for more information.

Class

The zone's name may optionally be followed by a class. If a class is not specified, class IN (for Internet), is assumed. This is correct for the vast majority of cases.

The hesiod class is named for an information service from MIT's Project Athena. It was used to share information about various systems databases, such as users, groups, printers, and so on. The keyword HS is a synonym for hesiod.

Another MIT development is Chaosnet, a LAN protocol created in the mid-1970s. Zone data for it can be specified with the CHAOS class.

Zone Options

allow-notify

See the description of allow-notify in the section called “Access Control”.

allow-query

See the description of allow-query in the section called “Access Control”.

allow-query-on

See the description of allow-query-on in the section called “Access Control”.

allow-transfer

See the description of allow-transfer in the section called “Access Control”.

allow-update

See the description of allow-update in the section called “Access Control”.

update-policy

This specifies a "Simple Secure Update" policy. See the section called “Dynamic Update Policies”.

allow-update-forwarding

See the description of allow-update-forwarding in the section called “Access Control”.

also-notify

This option is only meaningful if notify is active for this zone. The set of machines that receive a DNS NOTIFY message for this zone is made up of all the listed name servers (other than the primary) for the zone, plus any IP addresses specified with also-notify. A port may be specified with each also-notify address to send the notify messages to a port other than the default of 53. A TSIG key may also be specified to cause the NOTIFY to be signed by the given key. also-notify is not meaningful for stub zones. The default is the empty list.

check-names

This option is used to restrict the character set and syntax of certain domain names in zone files and/or DNS responses received from the network. The default varies according to zone type. For primary zones the default is fail; for secondary zones the default is warn. It is not implemented for hint zones.

check-mx

See the description of check-mx in the section called “Boolean Options”.

check-spf

See the description of check-spf in the section called “Boolean Options”.

check-wildcard

See the description of check-wildcard in the section called “Boolean Options”.

check-integrity

See the description of check-integrity in the section called “Boolean Options”.

check-sibling

See the description of check-sibling in the section called “Boolean Options”.

zero-no-soa-ttl

See the description of zero-no-soa-ttl in the section called “Boolean Options”.

update-check-ksk

See the description of update-check-ksk in the section called “Boolean Options”.

dnssec-loadkeys-interval

See the description of dnssec-loadkeys-interval in the section called “options Statement Definition and Usage”.

dnssec-update-mode

See the description of dnssec-update-mode in the section called “options Statement Definition and Usage”.

dnssec-dnskey-kskonly

See the description of dnssec-dnskey-kskonly in the section called “Boolean Options”.

try-tcp-refresh

See the description of try-tcp-refresh in the section called “Boolean Options”.

database

This specifies the type of database to be used to store the zone data. The string following the database keyword is interpreted as a list of whitespace-delimited words. The first word identifies the database type, and any subsequent words are passed as arguments to the database to be interpreted in a way specific to the database type.

The default is "rbt", BIND 9's native in-memory red-black tree database. This database does not take arguments.

Other values are possible if additional database drivers have been linked into the server. Some sample drivers are included with the distribution but none are linked in by default.

dialup

See the description of dialup in the section called “Boolean Options”.

delegation-only

This flag only applies to forward, hint, and stub zones. If set to yes, then the zone is treated as if it is also a delegation-only type zone.

See caveats in root-delegation-only.

file

This sets the zone's filename. In master, hint, and redirect zones which do not have masters defined, zone data is loaded from this file. In slave, stub, and redirect zones which do have masters defined, zone data is retrieved from another server and saved in this file. This option is not applicable to other zone types.

forward

This option is only meaningful if the zone has a forwarders list. The only value causes the lookup to fail after trying the forwarders and getting no answer, while first allows a normal lookup to be tried.

forwarders

This is used to override the list of global forwarders. If it is not specified in a zone of type forward, no forwarding is done for the zone and the global options are not used.

ixfr-base

This was used in BIND 8 to specify the name of the transaction log (journal) file for dynamic update and IXFR. BIND 9 ignores the option and constructs the name of the journal file by appending ".jnl" to the name of the zone file.

ixfr-tmp-file

This was an undocumented option in BIND 8. It is ignored in BIND 9.

journal

This allows the default journal's filename to be overridden. The default is the zone's filename with ".jnl" appended. This is applicable to primary (master) and secondary (slave) zones.

max-journal-size

See the description of max-journal-size in the section called “Server Resource Limits”.

max-records

See the description of max-records in the section called “Server Resource Limits”.

max-transfer-time-in

See the description of max-transfer-time-in in the section called “Zone Transfers”.

max-transfer-idle-in

See the description of max-transfer-idle-in in the section called “Zone Transfers”.

max-transfer-time-out

See the description of max-transfer-time-out in the section called “Zone Transfers”.

max-transfer-idle-out

See the description of max-transfer-idle-out in the section called “Zone Transfers”.

notify

See the description of notify in the section called “Boolean Options”.

notify-delay

See the description of notify-delay in the section called “Tuning”.

notify-to-soa

See the description of notify-to-soa in the section called “Boolean Options”.

pubkey

In BIND 8, this option was intended to specify a public zone key for verification of signatures in DNSSEC-signed zones when they were loaded from disk. BIND 9 does not verify signatures on load and ignores the option.

zone-statistics

See the description of zone-statistics in the section called “options Statement Definition and Usage”.

server-addresses

This option is only meaningful for static-stub zones. This is a list of IP addresses to which queries should be sent in recursive resolution for the zone. A non-empty list for this option internally configures the apex NS RR with associated glue A or AAAA RRs.

For example, if "example.com" is configured as a static-stub zone with 192.0.2.1 and 2001:db8::1234 in a server-addresses option, the following RRs are internally configured:

example.com. NS example.com.
example.com. A 192.0.2.1
example.com. AAAA 2001:db8::1234

These records are used internally to resolve names under the static-stub zone. For instance, if the server receives a query for "www.example.com" with the RD bit on, the server initiates recursive resolution and sends queries to 192.0.2.1 and/or 2001:db8::1234.

server-names

This option is only meaningful for static-stub zones. This is a list of domain names of name servers that act as authoritative servers of the static-stub zone. These names are resolved to IP addresses when named needs to send queries to these servers. For this supplemental resolution to be successful, these names must not be a subdomain of the origin name of the static-stub zone. That is, when "example.net" is the origin of a static-stub zone, "ns.example" and "master.example.com" can be specified in the server-names option, but "ns.example.net" cannot; it is rejected by the configuration parser.

A non-empty list for this option internally configures the apex NS RR with the specified names. For example, if "example.com" is configured as a static-stub zone with "ns1.example.net" and "ns2.example.net" in a server-names option, the following RRs are internally configured:

example.com. NS ns1.example.net.
example.com. NS ns2.example.net.

These records are used internally to resolve names under the static-stub zone. For instance, if the server receives a query for "www.example.com" with the RD bit on, the server initiates recursive resolution, resolves "ns1.example.net" and/or "ns2.example.net" to IP addresses, and then sends queries to one or more of these addresses.

sig-validity-interval

See the description of sig-validity-interval in the section called “Tuning”.

sig-signing-nodes

See the description of sig-signing-nodes in the section called “Tuning”.

sig-signing-signatures

See the description of sig-signing-signatures in the section called “Tuning”.

sig-signing-type

See the description of sig-signing-type in the section called “Tuning”.

transfer-source

See the description of transfer-source in the section called “Zone Transfers”.

transfer-source-v6

See the description of transfer-source-v6 in the section called “Zone Transfers”.

alt-transfer-source

See the description of alt-transfer-source in the section called “Zone Transfers”.

alt-transfer-source-v6

See the description of alt-transfer-source-v6 in the section called “Zone Transfers”.

use-alt-transfer-source

See the description of use-alt-transfer-source in the section called “Zone Transfers”.

notify-source

See the description of notify-source in the section called “Zone Transfers”.

notify-source-v6

See the description of notify-source-v6 in the section called “Zone Transfers”.

min-refresh-time, max-refresh-time, min-retry-time, max-retry-time

See the descriptions in the section called “Tuning”.

ixfr-from-differences

See the description of ixfr-from-differences in the section called “Boolean Options”. (Note that the ixfr-from-differences choices of master and slave are not available at the zone level.)

key-directory

See the description of key-directory in the section called “options Statement Definition and Usage”.

auto-dnssec

See the description of auto-dnssec in the section called “options Statement Definition and Usage”.

serial-update-method

See the description of serial-update-method in the section called “options Statement Definition and Usage”.

inline-signing

If yes, this enables "bump in the wire" signing of a zone, where an unsigned zone is transferred in or loaded from disk and a signed version of the zone is served, with, possibly, a different serial number. This behavior is disabled by default.

multi-master

See the description of multi-master in the section called “Boolean Options”.

masterfile-format

See the description of masterfile-format in the section called “Tuning”.

max-zone-ttl

See the description of max-zone-ttl in the section called “options Statement Definition and Usage”.

dnssec-secure-to-insecure

See the description of dnssec-secure-to-insecure in the section called “Boolean Options”.

Dynamic Update Policies

BIND 9 supports two methods of granting clients the right to perform dynamic updates to a zone, configured by the allow-update and update-policy options.

The allow-update clause is a simple access control list. Any client that matches the ACL is granted permission to update any record in the zone.

The update-policy clause allows more fine-grained control over which updates are allowed. It specifies a set of rules, in which each rule either grants or denies permission for one or more names in the zone to be updated by one or more identities. Identity is determined by the key that signed the update request, using either TSIG or SIG(0). In most cases, update-policy rules only apply to key-based identities. There is no way to specify update permissions based on client source address.

update-policy rules are only meaningful for primary zones (type master), and are not allowed in any other zone type. It is a configuration error to specify both allow-update and update-policy at the same time.

A pre-defined update-policy rule can be switched on with the command update-policy local;. named automatically generates a TSIG session key when starting and stores it in a file; this key can then be used by local clients to update the zone while named is running. By default, the session key is stored in the file /var/run/named/session.key, the key name is "local-ddns", and the key algorithm is HMAC-SHA256. These values are configurable with the session-keyfile, session-keyname, and session-keyalg options, respectively. A client running on the local system, if run with appropriate permissions, may read the session key from the key file and use it to sign update requests. The zone's update policy is set to allow that key to change any record within the zone. Assuming the key name is "local-ddns", this policy is equivalent to:

update-policy { grant local-ddns zonesub any; };

with the additional restriction that only clients connecting from the local system are permitted to send updates.

Note that only one session key is generated by named; all zones configured to use update-policy local accept the same key.

The command nsupdate -l implements this feature, sending requests to localhost and signing them using the key retrieved from the session key file.

Other rule definitions look like this:

( grant | deny ) identity ruletype [ name ] [ types ]

Each rule grants or denies privileges. Rules are checked in the order in which they are specified in the update-policy statement. Once a message has successfully matched a rule, the operation is immediately granted or denied, and no further rules are examined. There are 13 types of rules; the rule type is specified by the ruletype field, and the interpretation of other fields varies depending on the rule type.

In general, a rule is matched when the key that signed an update request matches the identity field, the name of the record to be updated matches the name field (in the manner specified by the ruletype field), and the type of the record to be updated matches the types field. Details for each rule type are described below.

The identity field must be set to a fully qualified domain name. In most cases, this represents the name of the TSIG or SIG(0) key that must be used to sign the update request. If the specified name is a wildcard, it is subject to DNS wildcard expansion, and the rule may apply to multiple identities. When a TKEY exchange has been used to create a shared secret, the identity of the key used to authenticate the TKEY exchange is used as the identity of the shared secret. Some rule types use identities matching the client's Kerberos principal (e.g, "host/machine@REALM") or Windows realm (machine$@REALM).

The name field also specifies a fully qualified domain name. This often represents the name of the record to be updated. Interpretation of this field is dependent on rule type.

If no types are explicitly specified, then a rule matches all types except RRSIG, NS, SOA, NSEC, and NSEC3. Types may be specified by name, including "ANY"; ANY matches all types except NSEC and NSEC3, which can never be updated. Note that when an attempt is made to delete all records associated with a name, the rules are checked for each existing record type.

The ruletype field has 16 values: name, subdomain, zonesub, wildcard, self, selfsub, selfwild, ms-self, ms-selfsub, ms-subdomain, krb5-self, krb5-selfsub, krb5-subdomain, tcp-self, 6to4-self, and external.

`name`	With exact-match semantics, this rule matches when the name being updated is identical to the contents of the `name` field.
`subdomain`	This rule matches when the name being updated is a subdomain of, or identical to, the contents of the `name` field.
`zonesub`	This rule is similar to subdomain, except that it matches when the name being updated is a subdomain of the zone in which the update-policy statement appears. This obviates the need to type the zone name twice, and enables the use of a standard update-policy statement in multiple zones without modification. When this rule is used, the `name` field is omitted.
`wildcard`	The `name` field is subject to DNS wildcard expansion, and this rule matches when the name being updated is a valid expansion of the wildcard.
`self`	This rule matches when the name of the record being updated matches the contents of the `identity` field. The `name` field is ignored. To avoid confusion, it is recommended that this field be set to the same value as the `identity` field or to "." The `self` rule type is most useful when allowing one key per name to update, where the key has the same name as the record to be updated. In this case, the `identity` field can be specified as `*` (asterisk).
`selfsub`	This rule is similar to `self`, except that subdomains of `self` can also be updated.
`selfwild`	This rule is similar to `self`, except that only subdomains of `self` can be updated.
`ms-self`	When a client sends an UPDATE using a Windows machine principal (for example, "machine$@REALM"), this rule allows records with the absolute name of "machine.REALM" to be updated. The realm to be matched is specified in the `identity` field. The `name` field has no effect on this rule; it should be set to "." as a placeholder. For example, `grant EXAMPLE.COM ms-self . A AAAA` allows any machine with a valid principal in the realm `EXAMPLE.COM` to update its own address records.
`ms-selfsub`	This is similar to ms-self, except it also allows updates to any subdomain of the name specified in the Windows machine principal, not just to the name itself.
`ms-subdomain`	When a client sends an UPDATE using a Windows machine principal (for example, "machine$@REALM"), this rule allows any machine in the specified realm to update any record in the zone or in a specified subdomain of the zone. The realm to be matched is specified in the `identity` field. The `name` field specifies the subdomain that may be updated. If set to "." or any other name at or above the zone apex, any name in the zone can be updated. For example, if update-policy for the zone "example.com" includes `grant EXAMPLE.COM ms-subdomain hosts.example.com. A AAAA`, any machine with a valid principal in the realm `EXAMPLE.COM` is able to update address records at or below "hosts.example.com".
`krb5-self`	When a client sends an UPDATE using a Kerberos machine principal (for example, "host/machine@REALM"), this rule allows records with the absolute name of "machine" to be updated, provided it has been authenticated by REALM. This is similar but not identical to ms-self, due to the "machine" part of the Kerberos principal being an absolute name instead of an unqualified name. The realm to be matched is specified in the `identity` field. The `name` field has no effect on this rule; it should be set to "." as a placeholder. For example, `grant EXAMPLE.COM krb5-self . A AAAA` allows any machine with a valid principal in the realm `EXAMPLE.COM` to update its own address records.
`krb5-selfsub`	This is similar to krb5-self, except it also allows updates to any subdomain of the name specified in the "machine" part of the Kerberos principal, not just to the name itself.
`krb5-subdomain`	This rule is identical to ms-subdomain, except that it works with Kerberos machine principals (i.e., "host/machine@REALM") rather than Windows machine principals.
`tcp-self`	This rule allows updates that have been sent via TCP and for which the standard mapping from the client's IP address into the `in-addr.arpa` and `ip6.arpa` namespaces matches the name to be updated. The identity field must match that name. The name field should be set to ".". Note that, since identity is based on the client's IP address, it is not necessary for update request messages to be signed. Note It is theoretically possible to spoof these TCP sessions.
`6to4-self`	This allows the name matching a 6to4 IPv6 prefix, as specified in RFC 3056, to be updated by any TCP connection from either the 6to4 network or from the corresponding IPv4 address. This is intended to allow NS or DNAME RRsets to be added to the `ip6.arpa` reverse tree. The identity field must match the 6to4 prefix in `ip6.arpa`. The name field should be set to ".". Note that, since identity is based on the client's IP address, it is not necessary for update request messages to be signed. In addition, if specified for an `ip6.arpa` name outside of the `2.0.0.2.ip6.arpa` namespace, the corresponding /48 reverse name can be updated. For example, TCP/IPv6 connections from 2001:DB8:ED0C::/48 can update records at `C.0.D.E.8.B.D.0.1.0.0.2.ip6.arpa`. Note It is theoretically possible to spoof these TCP sessions.
`external`	This rule allows named to defer the decision of whether to allow a given update to an external daemon. The method of communicating with the daemon is specified in the `identity` field, the format of which is "`local:``path`", where `path` is the location of a Unix-domain socket. (Currently, "local" is the only supported mechanism.) Requests to the external daemon are sent over the Unix-domain socket as datagrams with the following format: Protocol version number (4 bytes, network byte order, currently 1) Request length (4 bytes, network byte order) Signer (null-terminated string) Name (null-terminated string) TCP source address (null-terminated string) Rdata type (null-terminated string) Key (null-terminated string) TKEY token length (4 bytes, network byte order) TKEY token (remainder of packet) The daemon replies with a four-byte value in network byte order, containing either 0 or 1; 0 indicates that the specified update is not permitted, and 1 indicates that it is.

Multiple Views

When multiple views are in use, a zone may be referenced by more than one of them. Often, the views contain different zones with the same name, allowing different clients to receive different answers for the same queries. At times, however, it is desirable for multiple views to contain identical zones. The in-view zone option provides an efficient way to do this; it allows a view to reference a zone that was defined in a previously configured view. Example:

view internal {
    match-clients { 10/8; };

    zone example.com {
        type master;
        file "example-external.db";
    };
};

view external {
    match-clients { any; };

    zone example.com {
        in-view internal;
    };
};

An in-view option cannot refer to a view that is configured later in the configuration file.

A zone statement which uses the in-view option may not use any other , with the exception of forward and forwarders. (These options control the behavior of the containing view, rather than change the zone object itself.)

Zone-level ACLs (e.g., allow-query, allow-transfer), and other configuration details of the zone, are all set in the view the referenced zone is defined in. Be careful to ensure that ACLs are wide enough for all views referencing the zone.

An in-view zone cannot be used as a response policy zone.

An in-view zone is not intended to reference a forward zone.

Zone File

Types of Resource Records and When to Use Them

This section, largely borrowed from RFC 1034, describes the concept of a Resource Record (RR) and explains when each type is used. Since the publication of RFC 1034, several new RRs have been identified and implemented in the DNS. These are also included.

Resource Records

A domain name identifies a node. Each node has a set of resource information, which may be empty. The set of resource information associated with a particular name is composed of separate RRs. The order of RRs in a set is not significant and need not be preserved by name servers, resolvers, or other parts of the DNS. However, sorting of multiple RRs is permitted for optimization purposes: for example, to specify that a particular nearby server be tried first. See the section called “The sortlist Statement” and the section called “RRset Ordering”.

The components of a Resource Record are:

owner name	The domain name where the RR is found.
type	An encoded 16-bit value that specifies the type of the resource record.
TTL	The time-to-live of the RR. This field is a 32-bit integer in units of seconds, and is primarily used by resolvers when they cache RRs. The TTL describes how long a RR can be cached before it should be discarded.
class	An encoded 16-bit value that identifies a protocol family or an instance of a protocol.
RDATA	The resource data. The format of the data is type- and sometimes class-specific.

For a complete list of types of valid RRs, including those that have been obsoleted, please refer to https://en.wikipedia.org/wiki/List_of_DNS_record_types.

The following classes of resource records are currently valid in the DNS:

IN	The Internet.
CH	Chaosnet, a LAN protocol created at MIT in the mid-1970s. It was rarely used for its historical purpose, but was reused for BIND's built-in server information zones, e.g., `version.bind`.
HS	Hesiod, an information service developed by MIT's Project Athena. It was used to share information about various systems databases, such as users, groups, printers, etc.

The owner name is often implicit, rather than forming an integral part of the RR. For example, many name servers internally form tree or hash structures for the name space, and chain RRs off nodes. The remaining RR parts are the fixed header (type, class, TTL), which is consistent for all RRs, and a variable part (RDATA) that fits the needs of the resource being described.

The TTL field is a time limit on how long an RR can be kept in a cache. This limit does not apply to authoritative data in zones; that also times out, but follows the refreshing policies for the zone. The TTL is assigned by the administrator for the zone where the data originates. While short TTLs can be used to minimize caching, and a zero TTL prohibits caching, the realities of Internet performance suggest that these times should be on the order of days for the typical host. If a change can be anticipated, the TTL can be reduced prior to the change to minimize inconsistency, and then increased back to its former value following the change.

The data in the RDATA section of RRs is carried as a combination of binary strings and domain names. The domain names are frequently used as "pointers" to other data in the DNS.

Textual Expression of RRs

RRs are represented in binary form in the packets of the DNS protocol, and are usually represented in highly encoded form when stored in a name server or resolver. In the examples provided in RFC 1034, a style similar to that used in zone files was employed in order to show the contents of RRs. In this format, most RRs are shown on a single line, although continuation lines are possible using parentheses.

The start of the line gives the owner of the RR. If a line begins with a blank, then the owner is assumed to be the same as that of the previous RR. Blank lines are often included for readability.

Following the owner are list the TTL, type, and class of the RR. Class and type use the mnemonics defined above, and TTL is an integer before the type field. To avoid ambiguity in parsing, type and class mnemonics are disjoint, TTLs are integers, and the type mnemonic is always last. The IN class and TTL values are often omitted from examples in the interest of clarity.

The resource data or RDATA section of the RR is given using knowledge of the typical representation for the data.

For example, the RRs carried in a message might be shown as:

`ISI.EDU.`	`MX`	`10 VENERA.ISI.EDU.`
	`MX`	`10 VAXA.ISI.EDU`
`VENERA.ISI.EDU`	`A`	`128.9.0.32`
	`A`	`10.1.0.52`
`VAXA.ISI.EDU`	`A`	`10.2.0.27`
	`A`	`128.9.0.33`

The MX RRs have an RDATA section which consists of a 16-bit number followed by a domain name. The address RRs use a standard IP address format to contain a 32-bit Internet address.

The above example shows six RRs, with two RRs at each of three domain names.

Here is another possible example:

`XX.LCS.MIT.EDU.`	`IN A`	`10.0.0.44`
	`CH A`	`MIT.EDU. 2420`

This example shows two addresses for XX.LCS.MIT.EDU, each of a different class.

Discussion of MX Records

As described above, domain servers store information as a series of resource records, each of which contains a particular piece of information about a given domain name (which is usually, but not always, a host). The simplest way to think of a RR is as a typed pair of data, a domain name matched with a relevant datum and stored with some additional type information, to help systems determine when the RR is relevant.

MX records are used to control delivery of email. The data specified in the record is a priority and a domain name. The priority controls the order in which email delivery is attempted, with the lowest number first. If two priorities are the same, a server is chosen randomly. If no servers at a given priority are responding, the mail transport agent falls back to the next largest priority. Priority numbers do not have any absolute meaning; they are relevant only respective to other MX records for that domain name. The domain name given is the machine to which the mail is delivered. It must have an associated address record (A or AAAA); CNAME is not sufficient.

For a given domain, if there is both a CNAME record and an MX record, the MX record is in error, and is ignored. Instead, the mail is delivered to the server specified in the MX record pointed to by the CNAME. For example:

`example.com.`	`IN`	`MX`	`10`	`mail.example.com.`
	`IN`	`MX`	`10`	`mail2.example.com.`
	`IN`	`MX`	`20`	`mail.backup.org.`
`mail.example.com.`	`IN`	`A`	`10.0.0.1`
`mail2.example.com.`	`IN`	`A`	`10.0.0.2`

Mail delivery is attempted to mail.example.com and mail2.example.com (in any order); if neither of those succeeds, delivery to mail.backup.org is attempted.

Setting TTLs

The time-to- (TTL) of the RR field is a 32-bit integer represented in units of seconds, and is primarily used by resolvers when they cache RRs. The TTL describes how long an RR can be cached before it should be discarded. The following three types of TTLs are currently used in a zone file.

SOA

The last field in the SOA is the negative caching TTL. This controls how long other servers cache no-such-domain (NXDOMAIN) responses from this server.

The maximum time for negative caching is 3 hours (3h).

$TTL

The $TTL directive at the top of the zone file (before the SOA) gives a default TTL for every RR without a specific TTL set.

RR TTLs

Each RR can have a TTL as the second field in the RR, which controls how long other servers can cache it.

All of these TTLs default to units of seconds, though units can be explicitly specified: for example, 1h30m.

Inverse Mapping in IPv4

Reverse name resolution (that is, translation from IP address to name) is achieved by means of the in-addr.arpa domain and PTR records. Entries in the in-addr.arpa domain are made in least-to-most significant order, read left to right. This is the opposite order to the way IP addresses are usually written. Thus, a machine with an IP address of 10.1.2.3 would have a corresponding in-addr.arpa name of 3.2.1.10.in-addr.arpa. This name should have a PTR resource record whose data field is the name of the machine or, optionally, multiple PTR records if the machine has more than one name. For example, in the [example.com] domain:

`$ORIGIN`	`2.1.10.in-addr.arpa`
`3`	`IN PTR foo.example.com.`

Note

The $ORIGIN line in this example is only to provide context; it does not necessarily appear in the actual usage. It is only used here to indicate that the example is relative to the listed origin.

Other Zone File Directives

The DNS "master file" format was initially defined in RFC 1035 and has subsequently been extended. While the format itself is class-independent, all records in a zone file must be of the same class.

Master file directives include $ORIGIN, $INCLUDE, and $TTL.

The @ (at-sign)

When used in the label (or name) field, the asperand or at-sign (@) symbol represents the current origin. At the start of the zone file, it is the <zone_name>, followed by a trailing dot (.).

The $ORIGIN Directive

Syntax: $ORIGIN domain-name [comment]

$ORIGIN sets the domain name that is appended to any unqualified records. When a zone is first read, there is an implicit $ORIGIN <zone_name>.; note the trailing dot. The current $ORIGIN is appended to the domain specified in the $ORIGIN argument if it is not absolute.

$ORIGIN example.com.
WWW     CNAME   MAIN-SERVER

is equivalent to

WWW.EXAMPLE.COM. CNAME MAIN-SERVER.EXAMPLE.COM.

The $INCLUDE Directive

Syntax: $INCLUDE filename [ origin ] [ comment ]

This reads and processes the file filename as if it were included in the file at this point. If origin is specified, the file is processed with $ORIGIN set to that value; otherwise, the current $ORIGIN is used.

The origin and the current domain name revert to the values they had prior to the $INCLUDE once the file has been read.

Note

RFC 1035 specifies that the current origin should be restored after an $INCLUDE, but it is silent on whether the current domain name should also be restored. BIND 9 restores both of them. This could be construed as a deviation from RFC 1035, a feature, or both.

The $TTL Directive

Syntax: $TTL default-ttl [ comment ]

This sets the default Time-To-Live (TTL) for subsequent records with undefined TTLs. Valid TTLs are of the range 0-2147483647 seconds.

$TTL is defined in RFC 2308.

BIND Primary File Extension: the $GENERATE Directive

Syntax: $GENERATE range lhs [ttl] [class] type rhs [comment]

$GENERATE is used to create a series of resource records that only differ from each other by an iterator. $GENERATE can be used to easily generate the sets of records required to support sub-/24 reverse delegations described in RFC 2317: Classless IN-ADDR.ARPA delegation.

$ORIGIN 0.0.192.IN-ADDR.ARPA.
$GENERATE 1-2 @ NS SERVER$.EXAMPLE.
$GENERATE 1-127 $ CNAME $.0

is equivalent to

0.0.0.192.IN-ADDR.ARPA. NS SERVER1.EXAMPLE.
0.0.0.192.IN-ADDR.ARPA. NS SERVER2.EXAMPLE.
1.0.0.192.IN-ADDR.ARPA. CNAME 1.0.0.0.192.IN-ADDR.ARPA.
2.0.0.192.IN-ADDR.ARPA. CNAME 2.0.0.0.192.IN-ADDR.ARPA.
...
127.0.0.192.IN-ADDR.ARPA. CNAME 127.0.0.0.192.IN-ADDR.ARPA.

Both generate a set of A and MX records. Note the MX's right-hand side is a quoted string. The quotes are stripped when the right-hand side is processed.

$ORIGIN EXAMPLE.
$GENERATE 1-127 HOST-$ A 1.2.3.$
$GENERATE 1-127 HOST-$ MX "0 ."

is equivalent to

HOST-1.EXAMPLE.   A  1.2.3.1
HOST-1.EXAMPLE.   MX 0 .
HOST-2.EXAMPLE.   A  1.2.3.2
HOST-2.EXAMPLE.   MX 0 .
HOST-3.EXAMPLE.   A  1.2.3.3
HOST-3.EXAMPLE.   MX 0 .
...
HOST-127.EXAMPLE. A  1.2.3.127
HOST-127.EXAMPLE. MX 0 .

range	This can be one of two forms: start-stop or start-stop/step. If the first form is used, then step is set to 1. "start", "stop", and "step" must be positive integers between 0 and (2^31)-1. "start" must not be larger than "stop".
lhs	This describes the owner name of the resource records to be created. Any single $ (dollar sign) symbols within the lhs string are replaced by the iterator value. To get a $ in the output, escape the $ using a backslash \, e.g., \$. The $ may optionally be followed by modifiers which change the offset from the iterator, field width, and base. Modifiers are introduced by a { (left brace) immediately following the $, as in ${offset[,width[,base]]}. For example, ${-20,3,d} subtracts 20 from the current value and prints the result as a decimal in a zero-padded field of width 3. Available output forms are decimal (d), octal (o), hexadecimal (x or X for uppercase), and nibble (n or N for uppercase). The default modifier is ${0,0,d}. If the lhs is not absolute, the current $ORIGIN is appended to the name. In nibble mode, the value is treated as if it were a reversed hexadecimal string, with each hexadecimal digit as a separate label. The width field includes the label separator. For compatibility with earlier versions, $$ is still recognized as indicating a literal $ in the output.
ttl	This specifies the time-to-live of the generated records. If not specified, this is inherited using the normal TTL inheritance rules. class and ttl can be entered in either order.
class	This specifies the class of the generated records. This must match the zone class if it is specified. class and ttl can be entered in either order.
type	This can be any valid type.
rhs	rhs is an optionally quoted string.

The $GENERATE directive is a BIND extension and not part of the standard zone file format.

BIND 8 did not support the optional TTL and CLASS fields.

Additional File Formats

In addition to the standard text format, BIND 9 supports the ability to read or dump to zone files in other formats.

The raw format is a binary representation of zone data in a manner similar to that used in zone transfers. Since it does not require parsing text, load time is significantly reduced.

An even faster alternative is the map format, which is an image of a BIND 9 in-memory zone database; it can be loaded directly into memory via the mmap() function and the zone can begin serving queries almost immediately.

For a primary server, a zone file in raw or map format is expected to be generated from a textual zone file by the named-compilezone command. For a secondary server or for a dynamic zone, the zone file is automatically generated when named dumps the zone contents after zone transfer or when applying prior updates, if one of these formats is specified by the masterfile-format option.

If a zone file in a binary format needs manual modification, it first must be converted to a textual form by the named-compilezone command. Make any necessary modifications to the text file, and then convert it to the binary form via the named-compilezone command again.

Note that map format is extremely architecture-specific. A map file cannot be used on a system with different pointer size, endianness, or data alignment than the system on which it was generated, and should in general be used only inside a single system. While raw format uses network byte order and avoids architecture-dependent data alignment so that it is as portable as possible, it is also primarily expected to be used inside the same single system. To export a zone file in either raw or map format, or make a portable backup of such a file, conversion to text format is recommended.

BIND 9 Statistics

BIND 9 maintains lots of statistics information and provides several interfaces for users to access those statistics. The available statistics include all statistics counters that are meaningful in BIND 9, and other information that is considered useful.

The statistics information is categorized into the following sections:

Incoming Requests	The number of incoming DNS requests for each OPCODE.
Incoming Queries	The number of incoming queries for each RR type.
Outgoing Queries	The number of outgoing queries for each RR type sent from the internal resolver, maintained per view.
Name Server Statistics	Statistics counters for incoming request processing.
Zone Maintenance Statistics	Statistics counters regarding zone maintenance operations, such as zone transfers.
Resolver Statistics	Statistics counters for name resolutions performed in the internal resolver, maintained per view.
Cache DB RRsets	Statistics counters related to cache contents, maintained per view. The "NXDOMAIN" counter is the number of names that have been cached as nonexistent. Counters named for RR types indicate the number of active RRsets for each type in the cache database. If an RR type name is preceded by an exclamation point (!), it represents the number of records in the cache which indicate that the type does not exist for a particular name; this is also known as "NXRRSET". If an RR type name is preceded by a hash mark (#), it represents the number of RRsets for this type that are present in the cache but whose TTLs have expired; these RRsets may only be used if stale answers are enabled. If an RR type name is preceded by a tilde (~), it represents the number of RRsets for this type that are present in the cache database but are marked for garbage collection; these RRsets cannot be used.
Socket I/O Statistics	Statistics counters for network-related events.

A subset of Name Server Statistics is collected and shown per zone for which the server has the authority, when zone-statistics is set to full (or yes), for backward compatibility. See the description of zone-statistics in the section called “options Statement Definition and Usage” for further details.

These statistics counters are shown with their zone and view names. The view name is omitted when the server is not configured with explicit views.

There are currently two user interfaces to get access to the statistics. One is in plain-text format, dumped to the file specified by the statistics-file configuration option; the other is remotely accessible via a statistics channel when the statistics-channels statement is specified in the configuration file (see the section called “statistics-channels Statement Grammar”.)

The Statistics File

The text format statistics dump begins with a line, like:

+++ Statistics Dump +++ (973798949)

The number in parentheses is a standard Unix-style timestamp, measured in seconds since January 1, 1970. Following that line is a set of statistics information, which is categorized as described above. Each section begins with a line, like:

++ Name Server Statistics ++

Each section consists of lines, each containing the statistics counter value followed by its textual description; see below for available counters. For brevity, counters that have a value of 0 are not shown in the statistics file.

The statistics dump ends with the line where the number is identical to the number in the beginning line; for example:

--- Statistics Dump --- (973798949)

Statistics Counters

The following tables summarize the statistics counters that BIND 9 provides. For each row of the tables, the leftmost column is the abbreviated symbol name of that counter; these symbols are shown in the statistics information accessed via an HTTP statistics channel. The rightmost column gives the description of the counter, which is also shown in the statistics file, but, in this document, may be slightly modified for better readability. Additional notes may also be provided in this column. When a middle column exists between these two columns, it gives the corresponding counter name of the BIND 8 statistics, if applicable.

Name Server Statistics Counters

Symbol	BIND 8 Symbol	Description
Requestv4	RQ	This indicates the number of IPv4 requests received. Note: this also counts non-query requests.
Requestv6	RQ	This indicates the number of IPv6 requests received. Note: this also counts non-query requests.
ReqEdns0		This indicates the number of requests received with EDNS(0).
ReqBadEDNSVer		This indicates the number of requests received with an unsupported EDNS version.
ReqTSIG		This indicates the number of requests received with TSIG.
ReqSIG0		This indicates the number of requests received with SIG(0).
ReqBadSIG		This indicates the number of requests received with an invalid (TSIG or SIG(0)) signature.
ReqTCP	RTCP	This indicates the number of TCP requests received.
AuthQryRej	RUQ	This indicates the number of rejected authoritative (non-recursive) queries.
RecQryRej	RURQ	This indicates the number of rejected recursive queries.
XfrRej	RUXFR	This indicates the number of rejected zone transfer requests.
UpdateRej	RUUpd	This indicates the number of rejected dynamic update requests.
Response	SAns	This indicates the number of responses sent.
RespTruncated		This indicates the number of truncated responses sent.
RespEDNS0		This indicates the number of responses sent with EDNS(0).
RespTSIG		This indicates the number of responses sent with TSIG.
RespSIG0		This indicates the number of responses sent with SIG(0).
QrySuccess		This indicates the number of queries that resulted in a successful answer, meaning queries which return a NOERROR response with at least one answer RR. This corresponds to the success counter of previous versions of BIND 9.
QryAuthAns		This indicates the number of queries that resulted in an authoritative answer.
QryNoauthAns	SNaAns	This indicates the number of queries that resulted in a non-authoritative answer.
QryReferral		This indicates the number of queries that resulted in a referral answer. This corresponds to the referral counter of previous versions of BIND 9.
QryNxrrset		This indicates the number of queries that resulted in NOERROR responses with no data. This corresponds to the nxrrset counter of previous versions of BIND 9.
QrySERVFAIL	SFail	This indicates the number of queries that resulted in SERVFAIL.
QryFORMERR	SFErr	This indicates the number of queries that resulted in FORMERR.
QryNXDOMAIN	SNXD	This indicates the number of queries that resulted in NXDOMAIN. This corresponds to the nxdomain counter of previous versions of BIND 9.
QryRecursion	RFwdQ	This indicates the number of queries that caused the server to perform recursion in order to find the final answer. This corresponds to the recursion counter of previous versions of BIND 9.
QryDuplicate	RDupQ	This indicates the number of queries which the server attempted to recurse but for which it discovered an existing query with the same IP address, port, query ID, name, type, and class already being processed. This corresponds to the duplicate counter of previous versions of BIND 9.
QryDropped		This indicates the number of recursive queries for which the server discovered an excessive number of existing recursive queries for the same name, type, and class, and which were subsequently dropped. This is the number of dropped queries due to the reason explained with the clients-per-query and max-clients-per-query options (see the description about clients-per-query.) This corresponds to the dropped counter of previous versions of BIND 9.
QryFailure		This indicates the number of query failures. This corresponds to the failure counter of previous versions of BIND 9. Note: this counter is provided mainly for backward compatibility with the previous versions; normally, more fine-grained counters such as AuthQryRej and RecQryRej that would also fall into this counter are provided, so this counter is not of much interest in practice.
QryNXRedir		This indicates the number of queries that resulted in NXDOMAIN that were redirected.
QryNXRedirRLookup		This indicates the number of queries that resulted in NXDOMAIN that were redirected and resulted in a successful remote lookup.
XfrReqDone		This indicates the number of requested and completed zone transfers.
UpdateReqFwd		This indicates the number of forwarded update requests.
UpdateRespFwd		This indicates the number of forwarded update responses.
UpdateFwdFail		This indicates the number of forwarded dynamic updates that failed.
UpdateDone		This indicates the number of completed dynamic updates.
UpdateFail		This indicates the number of failed dynamic updates.
UpdateBadPrereq		This indicates the number of dynamic updates rejected due to a prerequisite failure.
RateDropped		This indicates the number of responses dropped due to rate limits.
RateSlipped		This indicates the number of responses truncated by rate limits.
RPZRewrites		This indicates the number of response policy zone rewrites.

Zone Maintenance Statistics Counters

Symbol	Description
NotifyOutv4	This indicates the number of IPv4 notifies sent.
NotifyOutv6	This indicates the number of IPv6 notifies sent.
NotifyInv4	This indicates the number of IPv4 notifies received.
NotifyInv6	This indicates the number of IPv6 notifies received.
NotifyRej	This indicates the number of incoming notifies rejected.
SOAOutv4	This indicates the number of IPv4 SOA queries sent.
SOAOutv6	This indicates the number of IPv6 SOA queries sent.
AXFRReqv4	This indicates the number of requested IPv4 AXFRs.
AXFRReqv6	This indicates the number of requested IPv6 AXFRs.
IXFRReqv4	This indicates the number of requested IPv4 IXFRs.
IXFRReqv6	This indicates the number of requested IPv6 IXFRs.
XfrSuccess	This indicates the number of successful zone transfer requests.
XfrFail	This indicates the number of failed zone transfer requests.

Resolver Statistics Counters

Symbol	BIND 8 Symbol	Description
Queryv4	SFwdQ	This indicates the number of IPv4 queries sent.
Queryv6	SFwdQ	This indicates the number of IPv6 queries sent.
Responsev4	RR	This indicates the number of IPv4 responses received.
Responsev6	RR	This indicates the number of IPv6 responses received.
NXDOMAIN	RNXD	This indicates the number of NXDOMAINs received.
SERVFAIL	RFail	This indicates the number of SERVFAILs received.
FORMERR	RFErr	This indicates the number of FORMERRs received.
OtherError	RErr	This indicates the number of other errors received.
EDNS0Fail		This indicates the number of EDNS(0) query failures.
Mismatch	RDupR	This indicates the number of mismatched responses received, meaning the DNS ID, response's source address, and/or the response's source port does not match what was expected. (The port must be 53 or as defined by the port option.) This may be an indication of a cache poisoning attempt.
Truncated		This indicates the number of truncated responses received.
Lame	RLame	This indicates the number of lame delegations received.
Retry	SDupQ	This indicates the number of query retries performed.
QueryAbort		This indicates the number of queries aborted due to quota control.
QuerySockFail		This indicates the number of failures in opening query sockets. One common reason for such failures is a due to a limitation on file descriptors.
QueryTimeout		This indicates the number of query timeouts.
GlueFetchv4	SSysQ	This indicates the number of IPv4 NS address fetches invoked.
GlueFetchv6	SSysQ	This indicates the number of IPv6 NS address fetches invoked.
GlueFetchv4Fail		This indicates the number of failed IPv4 NS address fetches.
GlueFetchv6Fail		This indicates the number of failed IPv6 NS address fetches.
ValAttempt		This indicates the number of attempted DNSSEC validations.
ValOk		This indicates the number of successful DNSSEC validations.
ValNegOk		This indicates the number of successful DNSSEC validations on negative information.
ValFail		This indicates the number of failed DNSSEC validations.
QryRTTnn		This provides a frequency table on query round-trip times (RTTs). Each nn specifies the corresponding frequency. In the sequence of nn_1, nn_2, ..., nn_m, the value of nn_i is the number of queries whose RTTs are between nn_(i-1) (inclusive) and nn_i (exclusive) milliseconds. For the sake of convenience, we define nn_0 to be 0. The last entry should be represented as nn_m+, which means the number of queries whose RTTs are equal to or greater than nn_m milliseconds.

Socket I/O Statistics Counters

Socket I/O statistics counters are defined per socket type, which are UDP4 (UDP/IPv4), UDP6 (UDP/IPv6), TCP4 (TCP/IPv4), TCP6 (TCP/IPv6), Unix (Unix Domain), and FDwatch (sockets opened outside the socket module). In the following table, <TYPE> represents a socket type. Not all counters are available for all socket types; exceptions are noted in the description field.

Symbol	Description
<TYPE>Open	This indicates the number of sockets opened successfully. This counter does not apply to the FDwatch type.
<TYPE>OpenFail	This indicates the number of failures to open sockets. This counter does not apply to the FDwatch type.
<TYPE>Close	This indicates the number of closed sockets.
<TYPE>BindFail	This indicates the number of failures to bind sockets.
<TYPE>ConnFail	This indicates the number of failures to connect sockets.
<TYPE>Conn	This indicates the number of connections established successfully.
<TYPE>AcceptFail	This indicates the number of failures to accept incoming connection requests. This counter does not apply to the UDP and FDwatch types.
<TYPE>Accept	This indicates the number of incoming connections successfully accepted. This counter does not apply to the UDP and FDwatch types.
<TYPE>SendErr	This indicates the number of errors in socket send operations. This counter corresponds to the SErr counter of BIND 8.
<TYPE>RecvErr	This indicates the number of errors in socket receive operations, including errors of send operations on a connected UDP socket, notified by an ICMP error message.

Compatibility with BIND 8 Counters

Most statistics counters that were available in BIND 8 are also supported in BIND 9, as shown in the above tables. Here are notes about other counters that do not appear in these tables.

RFwdR,SFwdR: These counters are not supported, because BIND 9 does not adopt the notion of forwarding as BIND 8 did.
RAXFR: This counter is accessible in the Incoming Queries section.
RIQ: This counter is accessible in the Incoming Requests section.
ROpts: This counter is not supported, because BIND 9 does not care about IP options.

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